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H: Convert perpetual magnet motor output to 5 volt DC
Disclaimer: Hi, I was in the neighborhood (StackOverFlow) and just noticed this cool site. I have had this general wondering for a while now and I am not sure if my question is constructive enough for this site (not from EE background). But I am genuinely interested to learn/know if there's any practical use of the concept. So, please do keep an open mind and any explanation would be greatly appreciated.
Concept: Let's say I have a DC motor (12V, 0.5A) and I managed to keep it spinning by "strategically" attaching some neodymium magnets around it (my intention is not to start a war regarding whether or not it is possible, but for the sake of practical usage, let's assume it is possible). The output I am getting is 7-9V, unknown amperage.
Question: Is it possible to convert this variable output to a stable 5.0 volt (standard USB output) with 0.5-1.0 amp to charge cellphone and if so, could anyone please provide some resources / explanation / diagram?
AI: Assuming we can completely ignore the laws of physics, especially as regards conservation of energy, then yes it is possible*.
The device you are looking for is called a "Voltage Regulator", and it takes in a variable voltage and outputs a stable, lower, voltage.
They come in two basic flavours:
Linear Regulator. These are like automatic variable resistors. They constantly adjust how much voltage is dropped across themselves in order to keep the output voltage stable. They are quite inefficient, and tend to get rather hot with higher currents.
Switching Regulator. These chop the incoming voltage up into small chunks by rapidly switching it on and off. The ratio of on to off gives the output voltage. A feedback loop monitors that output voltage and adjusts the ratio to keep it stable. A low-pass filter then smooths that on-off waveform into a single constant voltage again. These are much more efficient because they don't waste the excess voltage as heat, they just turn it off instead.
Now the electronics are sorted out, why not head over to Physics.SE and discuss perpetual motion and why it doesn't work in the real world. :)
* for certain values of "possible" |
H: NI-MH battery monitor design. Any obvious errors?
Let me preface this by saying I'm not that much of an EE expert, this is a "hobby" project, I consider myself as knowing enough to be dangerous. ;) I'd never apply for a full time career as an EE designer, that's for sure.
That said, I saw this: Low Current Battery Monitor Detail Question and this: Low current battery monitoring in regard to battery monitoring, but they don't address the situation I have.
In my case, I have a 6 cell NI-MH battery that is either not attached to the circuit at all, or it's attached and under load. I'm using a battery pack designed for an RC car, and to turn it off, I just disconnect it. However, I want a monitor that will indicate when the voltage drops below about 6V, since that's the point at which I want to consider the battery pack as discharged.
My current design is the following:
Please accept my apologies for the somewhat low quality circuit diagram, there's limits to what I can manage with a generic paint program.
The TI TL7757 is intended to assert /Reset if it's VCC drops below 4.55V, data sheet is here:
http://www.ti.com/lit/ds/symlink/tl7757.pdf
The Bivar 3BC-C-CA-F is a common anode two color LED, data sheet is here:
http://www.farnell.com/datasheets/1756451.pdf
My logic is that the 470 ohm pot will allow me to adjust the voltage on VCC of the TL7757 so that when the battery hits 6V, the TL7757 sees 4.55V.
Above that voltage, /Reset is high, it's an Open Collector output so the Red LED is off, however the leakage through it, the 270 Ohm resistor and the 100K resistor should be enough to turn on the 2N-3904, which in turn illuminates the green LED.
When /Reset drops, current flows though the red LED and the 270 Ohm resistor and the TL7757, illuminating the red LED, the voltage on the base of the 2N-3904 drops, turning it off and with it the green LED.
Have I made any mistakes that might stop this from working as intended?
AI: It looks pretty good, but there're a couple issues I can see.
One downside is you're just about doubling the current by using a 470 ohm pot for the detection part. I don't know how long you expect to have this circuit attached after the battery reaches the threshold for the red to turn on, but this circuit could excessively drain your batteries if you don't unplug it or charge it soon after seeing the red-light.
You should also make sure you're very careful when setting up/tuning your circuit. 1.2V * 6 = 7.2 which is higher than what this chip is rated for. I know you can get slightly higher than that with NiMH's so start at the low side when you're tuning the pot and work your way up. Another option is to add a resistor between the pot and the IC along with a 7V zener diode in parallel with the IC to protect it. Alternately, a higher Vdd IC could be sought. |
H: Motor controller diagram
I have been reading about motor controllers, and I found this article online (Introduction to Motor Drives and Encoders), but I am having a hard time understanding the diagram (figure below). Can someone explain the process of this control system in an algorithmic manner? I am really confused with the feedback part. I would appreciate any form of help.
AI: The intention of the diagram is to show that the feedback system compares the intended and actual speed of the motor and derives an error signal which is used to alter the power supply in such a way that the motor speed changes towards what it should be. Thusly:
The diagram is confusing in that it seems to imply that the output of the error comparator is amplified and used to directly control power supply voltage or perhaps frequency or ... in order to linearly control the motor based on the error amplitude.
In fact, what happens is the amplified error signal is used to alter the power supply output in a manner which will reduce the subsequent error. eg if the motor is running faster than desired the sum of desired-actual will be negative and the power supply will be controlled in such a way as to reduce speed. I purposefully avoided direct statements about exactly how the error signal is used to affect speed. Because ...
Depending on the motor technology used and load characteristics the process of altering motor speed may be very simple or extremely complex. In a simple DC motor drive the system may reduce motor voltage. In some systems it may increase field excitation. In a process which involves complex dynamic effects a traditional PID control algorithm or a custom fuzzy logic algorithm (or both or more) may be applied. |
H: Are all solenoids waterproof?
I have an application where I need a solenoid to control a valve at the bottom of a tank. For unimportant reasons the valve needs to be inside the tank at the bottom.
I was wondering are all solenoids inherently waterproof?
Current will follow the path of least resistance (kirchhoff's current laws prove this).
I did some basic internet searches and turned up that on the lowest end the resistivity of water is 20 Ohms per meter. If my solenoid is 24V and draws .6 amps that means the coil (plus added resistor) is 40 ohms.
Therefore the current would prefer to go through the water. I tested this theory by dropping my solenoid in my tank and running the power. It worked perfectly. But it should be noted my tank is plastic and is not grounded.
If I had grounded the water in the tank would my results have been different? or is there something else making the solenoid's coil have a lower resistance than the water in the tank?
AI: No they are not generally waterproof. The current will corrode the copper away over time and your coil will fail, even though the current flow will be much less than you think ( probably < a few mA ).
Even a small pinhole in the insulation of the copper wire will be enough to cause failure eventually.
If you get a solenoid with a molded sealed coil and waterproof the leads you should be able to make this work. Or, better, keep the electrics away from the water and run an actuator down into the water.
Edit: With regard to your specific question about "grounding the water in the tank", that does not make a lot of sense. The water is a bulk material with a relatively high resistivity. If you placed a metal plate within the tank and grounded the metal plate, and if the power to the solenoid was at some voltage with respect to ground, current would flow through the water. The details are actually quite difficult to calculate simply by hand because the electric field will not be simple. It's pretty easy to simulate with suitable software.
However, suppose you had a 1cm x 1cm x 10cm long rectangle of water with plates at either of the long ends and the resistivity of the water was 40 ohms-meter, then the current would be:
R= \$\rho L \over A \$ = \$40 \Omega-m \cdot 0.1m \over {0.01m \cdot 0.01m} \$ = 40k\$\Omega\$
so with 24V, 600uA would flow. This is kind of a "spherical horse in a vacuum" theoretical calculation but it should serve to illustrate that the currents involved will not be anything like the coil current (well, unless you get salt or other ionic contaminants dissolved in the water). |
H: How do I do basic circuit analysis with resistors rated in watts instead of ohms?
In my electrical circuits class, we've spent the last six weeks learning circuit analysis with resistors rated in ohms. Now, out of the blue, every question on the next practice exam has the resistors rated in the watts they consume. Here is a question from the practice exam where I'm supposed to find Ix.
Am I really supposed to find all of the node voltages by substituting the resistance values with V^2/P (or some manipulation of it) and then doing a loop current analysis? That gives a bunch of quadratic equations instead of making things easier.
I feel like I'm overlooking something mind-blowingly simple here. I've spent the last few hours searching for help on this and I can't find anything for this type of problem. Any help to point me in the right direction would save what little hair I have left on my scalp.
AI: The symbol for Ohms is the capital letter Omega, \$\Omega\$. In some word processors if you make the Omega symbol using a Greek font and then convert it to another font like Times New Roman or Arial, then that symbol will show up as a "W.". In other words, your professor probably used the wrong font and those are meant to be Omega's. |
H: Safe Lithium-ion Battery Management
I'm building a device that needs to run from Lithium-ion batteries.
I am well aware they need to be charged/dis-charged properly to avoid damage and explosions.
To my knowledge a charging system has two components which are the managed charging and cut-off once charged and discharge protection when the batteries voltage drops.
I found a board that uses the common TP4056 controller to charge the lithium-ion battery and cut-off once charged. It also has discharge protection at 2.5V.
Charging Unit
Lithium-ion Batteries
I have two questions:
So this means I can hook up the above (or any) Lithium-ion batteries to the charging board as well as my device and it would run well from the batteries until they got too low, at which the board would cut off the supply to the device leaving the device isolated from the batteries until charged again.
It says the discharge protection is at 2.5V isn't this well below the safe discharge voltage of a lithium cell. I thought you should never go below 3V?
I apologize if this question is rudimentary however I'd like to make sure when dealing with constant use of lithium-ion batteries.
AI: Batteries are tricky nonlinear devices. If you want to build a safe and reliable lithium battery charger, you need to know more about your batteries and your battery charger circuit. In general, lithium batteries are not interchangeable and not all chargers will work with all batteries.
There are also significant differences between lithium (probably not what you mean), lithium ion (probably what you mean, sometimes written Li+ or Li-ion), and lithium polymer (sometimes written LiPo) batteries, and significant differences within the battery chemistries of these categories.
The datasheet I found for TP4056 does not say that this device includes undervoltage protection. It only provides charging. Perhaps other circuitry does on that evaluation board does. The appropriate discharge protection is a function of battery chemistry, and the threshold should set according to the battery manufacturer's datasheet (as a baseline, anyway).
A circuit that tests for undervoltage is probably measuring the battery under load, and will need to compensate for the battery's internal resistance \$R_{internal}\$ reducing the voltage at the terminals \$V_t\$. That is
$$ V_t = V_{oc} - IR_{internal} $$
So to answer your second question it's quite possible that \$V_t\$ = 2.5V is an appropriate cutoff for a battery with \$V_{oc(min)}\$ = 3.0V, if \$ IR_{internal} \$ ~ 0.5V. |
H: Retrieving data recorded by an IoT sensor
If an IoT object records data (accelerometer, gyroscope, ...) in memory, what would be the smartest way to retrieve these data after, let's say, 2-3 days?
Store in JSON format on an SD Card?
Store in Memory then wire: USB?
Wireless: with BLE? XBee?
The idea is to retrieve the JSON data very easily from a computer without yet another hub, bridge, etc...
AI: If your IoT device can interface with an SD card, then sure, store the data as ASCII bytes, comma seperated, and you can do a very quick import into Micrsoft Excel or any other spreadsheet program. CSV (Comma seperated values) files are very easy to read with a Java program too, perhaps you need a quick custom Java program to convert them to be JSON compatible.
If your IoT device supports USB, you can make it store the data in Flash memory or similar non volatile (or volatile, your choice! if batteries will be good enough for that long) however you must look at the sample rate and data size and expected run time to see if your device actually has enough onboard storage. A very quick custom serial routine can be written to make it dump all the data over serial/USB after request, and you can write a very simple custom C, C++, Java, or C# program to receive and store as a local CSV or JSON format file (whatever you want, really).
If you always have something nearby you can indeed set up an Xbee serial link for remote data storage, or BLE based serial bridge to a nearby "base station" that can do the storage and monitoring. |
H: Smart ways to control an LED ladder?
I have two sets of LED ladders (12 in each ladder, so 24 LED's) that I want to control from my microcontroller. I don't have enough inputs for 24 LED's (and want to make it scalable), and the microcontroller has other tasks to attend to, as well as control the LED ladders.
I don't have a set amount of PIN's I can dedicate for a ladder, but the lesser the better (if a number is required, maximum of 8 pins per ladder)
The LED's will be updated very infrequently (every few hours), so I don't want to eat up processing power by multiplexing continously.
My microcontroller doesn't have a DAC
Currently, I am thinking of using shift registers (8 bit and 4 bit cascaded). However, since shift registers are "one way" devices I'll have to clear the registers to set a lower value.
I don't think this is a big problem (for me, as they get updated infrequently), but are there better methods to control an LED ladder?
Edit: By an LED ladder, I mean a single line of leds (like a bar of an equalizer) that shows a level of something. The LED's don't need to be controlled individually.
AI: If you want expandability, and to use almost zero Digital I/O pins for the purpose of merely LED driving, think about using a proper LED driver chip. You can do schenanigans with shift registers, but you will only go so far. If you are going to invest in board space for these ICs, why not just use a good old I2C bus LED driver with 16 channels each? Like the TLC59116 by Texas Instruments
Four hardware address pins allow up to 14 devices on the same bus. That gives you 14 * 16 total LEDs, all individually controllable (open drain, you can connect them straight to the IC with resistor from your voltage rail (up to 17V rated too!)
Not only are they individually controllable, but because it's an I2C bus you can add/remove any device from your "motherboard" in a very easy way (power + I2C connector, done!). Up to 224 LEDs with 8-bit PWM dimming, bus-wide commands, or individual LED commands. Quite amazing. Try it!
I should point out that by all means, try other types of multi-channel LED driver ICs, however the fact that there are I2C bus compatible ICs out there makes them EXTREMELY useful for expand-ability and quickly add/remove large amounts of LEDs, merely changing some software to deal with the hardware changes. The ICs are fairly cheap, only a few $ each but they will be better than trying to administer to many shift registers as fast as possible - let an IC deal with it, because that is what the are designed for.
Finally, you may use these more for "logic" than actual power driving, if the LEDs you are trying to drive are heavy duty (like, hundreds of mA to Amps each) by using inverting buffers and push/pull totem pole (also called cascode circuits) to operate the gate of a MOSFET (usually N channel, in a low-side power switch circuit). |
H: How much AWG do I need for 12V 20A
How much AWG do I need for my circuit wires knowing that the current can vary 10 to 20A with 12V supply
And what is the cheapest and safest way to join 2 wires together ?
AI: Using an online calculator which is based off some MIL standard for current carrying capacity of copper wires, given some parameters, I came up with 12AWG as the suitable wire gauge. This is approximately 2mm diameter core if you use metric system. This, given a 25 degrees Celsius ambient temperature and a wire length of 1 meter (2.5 feet) as some parameters, resulted in 23 Amp current carrying capacity.
These values are for a 10 degree Celcius rise in temperature in the conductor, and are part of the fitted curve of empirical test data some people took a long time ago for this sort of information which later went into the MIL Standard current carrying wire gauge charts.
If you can afford more temperature rise, you can get away with smaller cores, also if your peak current is only 20A, and not continuous.
If your application is sensitive to voltage drop - for example if you have a 10V regulator you really don't have much spare voltage headroom, every 100mV dropped can be bad for your 10V output. Be careful about the estimated cable resistance (given by the nice online calculator) and use Ohms law with the current you want, to estimate the voltage drop. If it's not acceptable, try larger wires or stop having things so far away ;). Or of course use a much higher voltage and step down later on, this is what electrical transmission grids do.
As for joining wires, using a screw socket terminal strip is pretty good! Maybe some shrink wrap to keep contacts from touching. This solution is very cheap, and can be found from almost any automotive supplies store (people often do this stuff for auto wiring, like trailer light harnesses etc) or hobby electronics shop. |
H: Connecting a surround system without jacks
I have recently bought a house which has already the sound cables passing through the walls. These are raw cables without any jack connectors.
My question is, when you go to buy a 5.1 surround system (or any speakers for that matter), what do you ask for when you do not want speakers with wires already connected.
Basically, when you have an AV receiver, there are jack inputs and there are also 'pure wired' inputs (where the cables are exposed). I want speakers which also accept this 'pure wired' inputs. I wish to know what these 'pure wired' inputs are called.
Thanks in advance!
AI: Most sound systems have speakes with terminals and custom cables with a custom connector at one end and bare wire at the other. Just connect the speakers to the existing wires as usual, and connect the custom wires to the other end with soldering or with a screw terminal. I also have this configuration in my living room and it works perfectly.
Anyway, even if the speakers are hardwired to the device, you can still cut the wires and connect them to the installed wires with screw terminals.
And I believe the phrase you're looking for is "speaker terminal". |
H: What is the use of magnets around electrical cables?
I haven't seen any for a while now, but a couples of year ago I saw some several times: cylindrical magnets around electrical wires. I'm not talking about specialized equipments but items of everyday life which comes with such a magnet. In fact I did not know it was a magnet until I accidentally broke one. Unfortunately I did not manage to find any picture from the web.
Do you know the use of them? Also, are we still using them?
AI: They're not actually magnets, but rather ferrite which is a paramagnetic material. A ferrite bead with a conductor through it is an inductor and so is used as a low pass filter. Typical use is for power cables to reduce EMI (electromagnetic interference). |
H: Indicated vs. true RMS value of sawtooth waveform
A sawtooth waveform is fed to "Average reading (with full scale rectification)" ac electronic voltmeter. This voltmeter is calibrated for RMS value of pure sinusoidal input. What will be reading displayed on voltmeter? Also find percentage error in the reading. Take amplitude as 10 volts and time period of 1 second.
The given answers are:
RMS indication = 6.5 V
error = 12.7%
Here's what I have done so far
\$V_{RMS}\$ of sawtooth is 5.77 V and average is 5 V. \$V_{RMS}\$ of sinusoid is 7.07 V and average is 6.37 V. So the ratio of \$V_{RMS}\$ to \$V_{avg}\$ is 1.11 and 1.154 of sinusoid and sawtooth respectively. But the question here is that voltage of sawtooth waveform is being measured by sinusoidally calibrated voltmeter and I don't understand how to find the value that will be measured and what will be the error.
AI: The key part in the question is that it's an "average reading (with full scale rectification)" rather than a true RMS calculation. What this means is that the input signal is rectified and the average value is then effectively multiplied by a constant with the assumption the input waveform was a pure sinusoid.
The definition of RMS for a periodic waveform is:
$$ V_{RMS} = \sqrt{\frac{1}{T}\int_0^T v^2(t) dt}$$
The average value with full scale rectification would be:
$$ V_{avg} = \frac{1}{T}\int_0^T |v(t)| dt$$
So the ratio of these for a pure sine wave would be the multiplication factor \$k\$ built into the meter. That is:
$$\begin{eqnarray}
k &=& \frac{V_{RMS}}{V_{avg}}\\
k &=& \frac{\sqrt{\frac{1}{T}\int_0^T v^2(t) dt}}{\frac{1}{T}\int_0^T v(t) dt}\\
k &=& \frac{\sqrt{\frac{1}{2\pi}\int_0^{2\pi} \sin^2(t) dt}}{\frac{1}{2\pi}\int_0^{2\pi} |\sin(t)| dt}\\
k &=& \frac{\frac{1}{\sqrt{2}}}{\frac{2}{\pi}}\\
k &=& \frac{\pi}{\sqrt{8}}\\
k &\approx& 1.11
\end{eqnarray}$$
In other words, the measured rectified average voltage will be multiplied by about 1.11 to get the indicated value.
To answer the first part of the question, you need to find \$V_{avg}\$ for the sawtooth waveform, which you have already correctly done and found it to be 5V.
What the meter will then do is multiply that by 1.11 and it will indicate 5.55V (not 6.5V -- your given answer is not correct, which was probably the source of your problem).
To determine the percentage of error is simply to compare that value with the true RMS value, which again, you have correctly calculated as 5.77V.
$$err = \frac{5.55 - 5.77}{5.77} = -0.0381 = -3.81\%$$
So the percentage of error is -3.81%, with the negative sign meaning that the indicated value is lower than the actual. Here again, the given answer was simply not correct.
If this is from a textbook, you might want to see if there are published errata that corrects that. If not, you might send the author a note -- although it's too late for you, it could save countless hours of frustration for future students. |
H: Did I do this circuit diagram correctly?
I just need some feedback on the circuit diagram I made (it's my first time):
This is for a mini-car. There are two battery packs, one connected to an Arduino UNO, and the other connected to two motors. Both battery packs have a SPST switch.
The battery pack connected to the motors has a 6V regulator, and an electromagnet connected to the Arduino (digital pin 8) has a 5V regulator.
Any feedback would be fantastic!
EDIT:
Okay, I changed some stuff around. Instead of connecting the motors/electromagnet directly to the digital pins (stupidly), I added MOSFET transistors to control the motors/electromagnet from the Arduino. The motors have a diode and capacitor. I hope this is a step in the right direction.
*the 6V regulator I mentioned earlier is not in here; I need to read more about it later and will add it in
EDIT2:
Okay, hopefully this is finalized circuit.
AI: I'm sorry to say, you need to go back to the drawing board.
An Arduino Uno (i.e., ATMega328P) can only sink or source an absolute maximum of 40mA through any one of its IO pins, and Atmel only guarantee up to 20mA.
That is not enough current to power (or sink power from) things like motors, electromagnets, etc.
You need to switch the motors etc with transistors, and add flyback diodes to absorb the induced back EMF from the collapsing magnetic fields.
Treat them like you would a relay - google "Arduino Relay" for how to do that.
Also, your regulators make no sense, and your batteries are backwards. |
H: Protecting a circuit from the effects of a capacitor's charging
During its first outing, I managed to burn out the (control circuitry) of two of the batteries connected to my LED jacket (see this, this and this question for context.) I thought I should have been well within spec for the total current I was drawing, so I'm now wondering if it wasn't the total, but just how quickly I was trying to draw it (the draw is quite "peaky".)
In order to mitigate this in the next iteration of my design, I was thinking I could put a few hefty (10,000µF?) capacitors across my power bus. But I understand that they will draw a large amount of current themselves while charging. Can I prevent this by putting a resistor in series with each capacitor?
What resistor values would be appropriate if the voltage across the bus is 5V and I am using four 10,000µF capacitors? Or is there a better way to limit the current drawn by the capacitors on startup?
AI: My money is the fact that the "batteries" are in parallel.
In actual fact, what you have there aren't just batteries, but batteries with boost converters.
If you open them up you will find a LiPo battery at (around) 3.7V. That feeds a boost converter to raise the voltage to 5V.
Putting voltage regulators (of any sort) in parallel is never a good idea as they will basically fight each other for control over the target voltage, resulting in nasty things happening.
You would be better off splitting your system into separate power zones with different groups of LEDs powered off different batteries. That way they only share a common ground, not common power. |
H: Is there any offline circuit simulator software that can simulate a noisy power supply?
I have been using Circuit Wizard (Its the only offline program that I can find!). Unfortunately, it is really basic, and I have outgrown it.
I need to simulate a 5V circuit with a noisy power supply, with a lot of ripple/noise (anything from 0.1v to 1v). I am an electronics novice, and I have tried all sorts of things, like these:
And none of them work properly. I was wondering:
Is there a way to make this work at all?
Is there a good offline circuit simulator (as I do not always have internet access) that is easy to use, and can simulate noisy power supplies?
EDIT: I do not mind if the programs are freeware or paid.
AI: You need to put your noise source in series with the ideal source:
simulate this circuit – Schematic created using CircuitLab
Voltage sources in series add. (Ideal) voltage sources in parallel are a logical contradiction. |
H: How do I read this relay pinout/schematic?
I'm trying to understand the way this relay works. To me it looks like this KUHP-11DT1-12 is a DPDT relay. Pin 7 is connected to 1 or 4 and pin 9 is connected to 3 or 6 depending on whether the coil is energized or not.
What I cannot figure out from looking at the datasheet or the diagram is which is normally open and which is normally closed. If I had the part in hand, I could simply ohm it out, but at this point I don't yet have the device.
How can I tell from the datasheet instead of having to ohm it out?
Product Data Sheet
AI: A Form A relay contact is Normally Open, so since the data sheet says to delete terminals 1 and 3 for a Form A relay, those must be the Normally Closed contacts, and 4 and 6 are then the Normally Open contacts. |
H: Integrating simple Bluetooth functionality without microcontrollers
I have recently bought a Bluetooth HC-05 Module, and have followed many tutorials on integrating it with an Arduino, or a microcontroller. However I got interested in whether it is possible without one. And the task I need to achieve is pretty simple, just switching an LED on/off. The official bluetooth documentation is impossible to read, so I'm asking the question here, since I know many of you are experts in the subject.
AI: Think of all the interfacing that needs to be done. The module can't put a pin high when you send something to it, or low when you send something else. It's much more advanced, and involves communication standards that use timing. So if you would do this with 'simple components' (logic ports and the like) it would get extremely advanced, and you would simply have a little microcontroller. |
H: Wiring up an LCD display with unknown controller
I found an old LCD in my pile of trunk. I would like to be able to use it. However, someone (possibly / probably me) sawed off the part of the PCB with the series code, so I can't know what the controller chip is. I'm trying to figure it out.
My analysis so far:
The display has backlight. There are 16 pins, so HD44780 would be my first guess (3 power pins, 11 data pins and 2 for backlight). The three pins that are wired up could be power, looking at the traces, and the last two LED anode and cathode. So I tried to wire it up with pin 1 to 0V, pin 2 to 5V, and pin 3 (probably contrast) to 0V. I got this result:
This looks like HD44780 to me, with a 2x16 screen.
But I've got a few questions about this:
Are there other (not HD44780-compatible) displays that would give this display when only power is supplied (but no controls), or is this specific for HD44780?
The pinout I have used up to now for all LCDs with HD44780 I've used so far (as far as I can remember) had a pinout like this:
Power (Vss, Vdd, Vcontrast)
Control (RS (register select, command or data), R/W (read / not write), E (enable)
Data (DB0 - DB7)
Possibly backlight (Anode, Cathode)
But is this a standard? Can I assume that this uses the same pins for the same functions? And, if not, is it safe to just try it, or could things go wrong? If it could go wrong, is there a way to say judging by the PCB traces what lines would be data, probably?
I don't think it's important, but in the end I intend to use this chip with a PIC. I have used HD44780 displays with PIC before, so I have working code to try out (and also other displays with HD44780 (or ~compatible) to check the code and setup).
AI: Are there other (not HD44780-compatible) displays that would give this display when only power is supplied (but no controls), or is this specific for HD44780?
This is not specific for the HD44780 or similar controllers, but as a matter of fact most character lcds are HD44780 compatible. You can take it for granted.
The pinout I have used up to now for all LCDs with HD44780 I've used so far (as far as I can remember) had a pinout like this snip But is this a standard?
A defacto standard, there can be variations, but again, market forces have consolidated that 14/16 pin straight header as pinout. There is also the Dual Inline pinout.
Can I assume that this uses the same pins for the same functions? And, if not, is it safe to just try it, or could things go wrong? If it could go wrong, is there a way to say judging by the PCB traces what lines would be data, probably?
Based on the looks, yes, go ahead it should work. You already verified the power pins, and the backlight pins. The rest look like datalines into the Chip on Blob. They are all Thin, with no caps or resistors on them. |
H: Joining a raspberry Pi to an external hard disk cage power supply
I have an external SATA hard drive enclosure that has its own power input. I would like to be able to run a Raspberry Pi from the same power input.
The enclosure takes a 12VDC input (currently I'm using a 1.5A power supply but I'm sure I have a 2A one lying around somewhere) and converts it to a SATA power input for the drive. The hard drive says it wants 0.65A from the 5V on the SATA power, and the RPi draws 1A from its 5V input at most. The intended setup is for a small NAS so the Pi will be using its ethernet port on the usb hub chip but no peripherals after the original setup (which can be done when powered normally).
So, is it possible/sensible to split the 5V cable on the SATA power and hook it up to a µUSB to power the Pi, bearing in mind the variable power consumption of the drive and Pi?
If not, why?
AI: It might work, but I wouldn't recommend it. Purely because the components in the power system for the HDD enclosure will be sized for running a hard drive. The controller board will include a 5V regulator, and a 3.3V regulator. That 5V regulator may not be able to reliably provide both the power for the HDD and the Pi.
Instead I would recommend adding a second power board that takes power direct from the 12V input and converts it to a separate 5V supply just for the Pi. I'd recommend a switching power supply, and the simplest form would be a UBEC (Universal Battery Elimination Circuit) used in many RC vehicles, and available from most RC stores both online and otherwise. |
H: Capacitors vs buck/boost converters
I'm trying to drive high power LEDs with a laptop power supply, and wondered if I should use resistors to limit the extra current, or something like this to limit the current. If I did use that module, I would run it at 55V @ .7A
I'm aiming for decent efficiency without designing my own module. The laptop power supply I'm using is rated at 19.6V @ 4.62A. From there, I could either use a (cheaper) boost converter like this, which only limits voltage, and from there use resistors to limit the current. Or I could use the more expensive first linked module to regulate current and voltage. Which is better, in terms of efficiency?
I feel like the boost converter would be more efficient, but everyone else's circuits use resistors to limit current. Is there something I'm missing?
If I've made any incorrect assumptions, please tell me. I want to make sure I do this well.
AI: Use a current regulator.
A voltage regulator won't do as LEDs change their resistance when they get warm and thus needs different voltage to get the same output from them. If you use a resistor you will just burn Watts and make a nice oven, even at 700mA.
current regulators are the way to go, don't cost too much and will be most efficient as you don't need any resistors and.
And if you want to be very efficient, select a switching current regulator...
It would be also a good idea to have the input voltage as close as possible to the LED(s) voltage. |
H: A simple RL circuit, getting transfer function, matching with results
I'm doing little lab exercise with an RL circuit. I need to check if I did this analysis right, because the numbers I got and the numbers that should theoretically be there are so far off, I figured I had to be doing something wrong.
We have the following circuit:
simulate this circuit – Schematic created using CircuitLab
And we want to measure the voltage across AB and build a bode plot, and ID what kind of filter it is.
So I get a bunch of values for the voltage. No problem, I plot them and the shape looks like a good Bode plot. I even marked the cutoff frequency and it looked like it was in the right place.
But when I tried deriving a theoretical Bode plot the curve always looked off -- way off. It was either too far to the right (the inflection point was off by orders of magnitude) or too high or too low. So I want to make sure I derived the response function correctly. If yes, then fine; I can say that it's off because the resistor and inductor aren't perfect. (One would expect that). But I am really struggling here.
So, here's what I did: the response function on this circuit will be
$$H(\omega)=\frac{V_{out}}{V_{in}}$$
\$V_{in}\$ is easy enough since I set the RMS at 1 V. But mathematically it should be (Ohm's law) V=IR and the total impedance of the circuit at a given frequency (using f not omega) is \$R+ i\frac{f}{2\pi}L)\$. That means \$I = \frac{V_{in}}{R+ i\frac{f}{2\pi}L}=\frac{2\pi V_{in}}{2\pi R+ ifL}\$. That should mean that
$$V_{out} = V_{AB} = \frac{V_{in}fL}{2\pi R+ ifL}$$
and my response function is $$H(f) = \frac{fL}{2\pi R+ ifL}$$
I need to convert this to a real number and a magnitude, so I did this:
$$|H(f)|^2 = \frac{(fL)^2}{(2\pi R+ ifL)(2\pi R- ifL)}=\frac{(fL)^2}{(2\pi R)^2+ (fL)^2}\rightarrow |H(f)| = \frac{(fL)}{\sqrt{(2\pi R)^2+ (fL)^2}}$$
But this does not produce anything like the values I got. The Bode plot goes up to a certain value and flattens out, as though this were a low pass and not a high pass filter.
Now, if I were using just R in the numerator of my transfer function it looks the right shape, but not in the right place.
Her'es the values in millivolts I got for \$V_{AB}\$ at various multiples of fc (cutoff frequency) in Hz. Fc = 4289 Hz.
428.9: 68.3
2144.5: 64.8
4289: 52.6
8578: 34.7
17156: 1.2
Anyhow I am just trying to suss out what I did wrong here, if anything.
AI: First of all, the circuit drawn is a high-pass filter.
At very high frequencies, the inductor has very high impedance and thus, the output \$V_{ab}\$ should essentially equal the input.
At very low frequencies, the inductor has very low impedance and thus, the output should be essentially zero.
The frequency where the output voltage is \$\frac{1}{\sqrt{2}}\$ times the input voltage is given by
$$f_0 = \frac{R}{2\pi L} $$
So, either the circuit drawn does not reflect the circuit measured or something went terribly wrong with your measurement apparatus (or operator of said apparatus).
For completeness, the transfer function is, by inspection:
$$H(j\omega) = \frac{j\omega L/R}{1 + j\omega L/R}\quad,\quad\omega = 2\pi f$$
$$|H(f)| = \frac{2\pi fL/R}{\sqrt{1 + (2\pi fL/R)^2}} $$ |
H: A-D conversion: effect of adding more bits
Say you have a digitizer that's converting an analog signal and feeding it to a microprocessor for whatever you'd like to do to it. How would increasing the number of bits of the digitizer affect the signal? I feel like it would have something to do with the resolution of the signal but that's just a guess and I have no idea how to explain this concept. Also, I feel like there should be a mathematical relationship between these two but I don't think I've seen it anywhere else online.
Additionally, how would the sampling frequency that was first used to obtain the analog signal be related to the number of bits. Are there any precautions that should be taken?
AI: Whenever a continuous signal is encoded into a finite number of bits, there will be some quantizing noise. This is the error introduced because not all possible input values can be accurately represented by the digitized signal. For example, assume that an analog signal ranging from 0V to 5V is digitized using a 2-bit ADC. Using equal step sizes, this might be done as shown in the table below:
$$
\begin{array}{cccc}
{\bf voltage} & {\bf binary} & {\bf code} & {\bf output} \\
0 \le v < 1.25 & 00 & 0 & 0.0 \\
1.25 \le v < 2.50 & 01 & 1 & 1.25 \\
2.50 \le v < 3.75 & 10 & 2 & 2.50 \\
3.75 \le v < 5.00 & 11 & 3 & 3.75 \\
\end{array}
$$
If voltage of 2.0V is digitized, it is encoded as 01 binary, but so would voltages of 1.26V or 2.48V. If we convert back to analog, all of them would result in an output voltage of 1.25V according to the scheme above. The difference between the sampled voltage and the corresponding output is quantizing noise.
It's easy to see that adding bits adds resolution and reduces quantizing noise.
As for sampling frequency, there is a thing called the Nyquist-Shannon sampling theorum that states to be able to successfully reproduce any digitized signal, one must sample at no less than twice the highest frequency you wish to reproduce from the input. This is independent of the number of bits one uses to digitize. |
H: PCB design review: Decoupling, bypassing and grounding
I'm trying to make my own wireless sensor using ATmega microcontroller. I've some basic understanding of electronics but I'm a total amateur when it comes to designing circuits and PCBs.
This is just a hobby project so it's not expected to be of a production quality or anything like that. But still I would like to make it as good as possible just for a good feeling of doing things right.
I've read something about decoupling, bypassing and grounding but not sure if I understand it correctly. So if someone could look at the board and point out things which should be done differently I would be really thankful.
Edit: added schematic of the circuit
AI: Etching PCBs at home is fun, but the process makes PCB design a bit tricky for hobbyists like us. The most important constraint is that it's difficult to make plated through-holes. And those are essential for soldering components to double-sided boards like yours.
If you have your board made at a fab house, you'll get all of your PCB vias and holes plated like in the picture below. That's nice because you can then solder all components on the bottom of your board and be done with it. That's possible because all tracks on top will be linked to the bottom layer at the plated holes, thus ensuring continuity.
(Picture from RobotRoom.com)
If you don't master some kind of through-hole electroplating at home (a process that involves nasty chemicals), you won't get those holes plated. The end result is that you'll have to solder the component pads on the top of your board whenever there's a track reaching them.
Soldering on the top layer can be inconvenient at best, or just impossible, depending on the component and the PCB design. For example, headers are easy to access from the bottom, but they usually have a plastic support that gives them mechanical strength and that sits flush on top of the board. DIP sockets also brings problems as they sit flush on top as well.
Solder components on the top layer is usually a bad idea also because it hinders maintenance. You may assemble your board in an order that lets you access hard to reach pads, but when all the components are placed, you may be left without access to those pads. In my experience, because these pads are harder to reach, the soldering quality suffers and these pads end up being the first to fail.
To work around those problems, I suggest you do the following:
Design single-sided boards whenever you can. Use your schematic as an exercise, for example. Try to place its components carefully so that they make single-sided routing easier and try and route all tracks that you can on the bottom layer. I usually change pin assignments if it makes routing easier. If you need to cross tracks, use plain jumpers (I usually use remains of through-hole components for that). I use the top layer to represent the jumpers. Also, plan to use resistors and capacitors as jumpers. I've done just that in the picture below, on ATmega328P pins 2 and 3 - serial TX & RX.
If you can't make a single-sided board for some reason, make sure your tracks only change sides at vias, like in the places marked in orange in the picture below, and not at the component pads. You can then solder a piece of wire on both sides of your board on those vias. This will make for a much more robust solution than soldering at the components pads.
Make the vias really large (I use 0.07 inch or ~1.8mm). If you follow rule #2 above and only make tracks change sides at vias, those large vias are the only features you'll have to match between both sides of your board. Large vias make the registration process of double-sided boards easier.
I also try to stick to the 0.05 inch grid on Eagle, make only 45° turns and leave as much space as you can from tracks and pads of different signals.
Considering your design and the fact that you will etch it at home, and assuming that you can't make plated trough-holes, you'll have trouble with your oscillator crystal and all of your connectors. I would suggest you use the trick of bringing the track away from the pads into a via and then cross it over to the other side for those components.
You may get away with soldering the ceramic capacitors and resistors on the top layer, but I don't recommend that. I'd just re-route your board to try and bring most, if not all tracks to the bottom layer.
I don't know whether there are issues with the external components you plan to use (the NRF905 for example), but I would watch it if they use high-frequency signals for driving them (anything above 500kHz).
Below is my (quick and dirty) attempt at a single-sided version of your board. Pardon me if I made any mistakes, but it should give you an idea of how to try and route most of the tracks on the bottom layer. I had to use 5 jumpers, but you may be able to do better.
I now realize that my version of the 14-pin connector is mirrored in relation to yours, which made my version easier to route. But, if you don't have any software or hardware restriction, you may reassign the MCU pins to the connector so that it is easier to route your version as well.
I hope this helps. |
H: Creating an isolated power supply to prevent effects of lightning
Transformers isolate the load completely from the primary power source. Does that mean that I can connect two or more transformers one after the other and expect my Linear Power Supply to have better safety than having the isolation of one transformer?
For example, lets assume I need 12V AC out. The situation I am trying to imagine here is having 230V/50V and 50V/12V transformers.
Would this make my system more immune to lightning?
Are there better ways of doing the same?
AI: Tranformers can protect against lightning in two ways- first they have galvanic isolation from primary to secondary that is good for hundreds, and more likely, thousands of volts of transient voltage. That's good for common-mode voltage (voltage on the input leads wrt earth). Secondly, they saturate and will prevent high normal mode (across the input leads) transients which may be induced by a lightning discharge from reaching the circuit on other side (they might let double their rated voltage through, but not 10x or 100x).
The problem with the galvanic isolation is that it may not be enough for a direct lighting strike. It's thus better to find some way to conduct the discharge to ground through a lightning arrestor (basically a spark gap) so that the voltage doesn't rise too high. For transients on telecom lines, very small spiral gas discharge tubes as "Enemy Of the State Machine" mentions are available.
The way to deal with such a huge amount (potentially) of energy is to divert, then soak it up in some series impedance, divert again, and some more series impedance until what's left is harmless. If you try to divert or absorb it all with some small device, the end result will be a couple of disembodied smoking leads, and no small device any more. In many cases there will be a natural spark gap in the creepage and clearance distances in your input plug, switch, terminal blocks etc. but it's not always wise to depend on those, as they may break over after your transformer does. If your "first in line" transformer breaks over to ground internally, it will probably be ruined (assuming it 'tracks' through the insulation), so your second transformer may save the circuit but a service call is still required to replace the transformer. |
H: COTS solutions for ±18V DC (or even ±20V) and ±30V PCB-mounted power supplies in the 3-5 Watt range
Most manufacturers make DC-DC converters with only ±15V or ±24V outputs for driving opamps. Opamps generally fall in several max-DC-supply classes. Typical are the ±22V ones (e.g. LM741 and a ton more), some of which do have reliable operation up to ±20V (e.g. the LM741A). Alas DC-DC converters in between ±15V and ±24V (e.g. ±18V) are extremely boutique items than can be very hard to source. So, what are practical solutions for building your own (on par with COTS devices in terms of characteristics) wile using as few components as possible? A ±9V with voltage doublers for example is a practical solution?
The same question applies to ±30V sources that would be useful for getting most output swing out of an OPA551 for example (which has its characteristics given at this supply voltage, by the way.)
AI: Assuming you're doing the sort of thing that can live without galvanic isolation, there are plenty of easy solutions. If you don't mind a transformer, here is one of the lowest parts-count (and probably lowest cost) solutions, using the ubiquitous LM2577:
The simpler solutions (as with the cheaper DC-DC converters) tend to have so-so regulation, require a minimum load etc. Be mindful of this if you're up near the absolute maximum voltage input of the load semiconductors- the output voltage could be volts above nominal with a very light load. |
H: Solderless breadboards: standard size specifications?
Are there standard sizes for solderless breadboards?
For example, My teaching project is currently reaching the capacity limit of a 170 pin (17 x 10) breadboard. Is there a standard "next size larger" I can specify?
AI: There are various de facto standard sizes.
Tiny (170 tie points)
Half-sized (400 tie points)
Full-sized (830 tie points)
Larger (with or without doubled power rails)
Note that spacing from breadboard to breadboard over a single rail is 700mil. |
H: How to Calculate Mosfet Voltages
For a N-channel Mosfet does Vds = Vdb + Vbs? Is this only true for simple models or always?
Where d is drain, s is source and b is base.
AI: B is body on a MOSFET, not base. The relation you posted is true regardless of model - it's how the pin voltages are defined.
The voltages of the source, drain, gate, and body pins are Vs, Vd, Vg, and Vb. However, the absolute voltages are not important - the differences are. So the differences are defined as Vgs = Vg - Vs, Vds = Vd - Vs, Vgb = Vg - Vb, etc.
Following this convention, Vdb = Vd - Vb and Vbs = Vb - Vs so Vds = Vd - Vb + Vb - Vs = Vd - Vs. |
H: How to make LC tank circuits oscillate continuously?
An LC tank circuit produces oscillations, when the capacitor is charged and connected to the inductor, but the oscillations decay gradually. How to make it continue, i.e. oscillate continuously?
I know we need to add energy continuously, but I don't know how.
Could anyone describe a circuit that provides a non-decaying oscillation?
AI: simulate this circuit – Schematic created using CircuitLab
Here is an LC Oscillator that is easy to understand.
All the values are examples only.
However, it is necessary to select the four equal resistors fulfilling the condition R2/R1=R4/R3.
In theory, you could drop the resistor R3 - however, in this case you should know the damping properties (that means the parallel loss resistance Rloss) of the tank circuit.
Because this is not possible it is wise to heavily damp the LC circuit with R3 because,in this case, the losses of the componenets L and C are "overshadowed" and play no role.
Thus, you have an opamp with two feedback pathes (positive and negative) which cancel each other for one single frequency only: The resonant frequency of the tank which gives the oscillation frequency.
Remark: For a safe start of oscillations it is necessary to slightly increase the non-inverting gain (1+R2/R1) above its nomional value. That means: R2/R1>R4/R3. |
H: Why LED screens have to twinkle?
I was researching in psychophysic on optic experiment and I stumble upon my laptop manufacturer'screen specs. There is a 60hz refresh rate with exponentially decaying lums intensity between each tick.
Wouldn't it be visually nicer if the digital video signal were converted to continuous analogical first and then feed each led continuously as it happen in audio transducer ?
AI: It's just not feasible. In a full HD display panel, there are 1920 x 1080 x 3 = 6,220,800 pixels. That would be over 6 million wires to connect each one to a drive circuit. And 6 million pins on the panel. And 6 million drivers. It's really not possible to build. So what they do instead is multiplex - row and colum wires with a pixel at each crossing. This brings down the number of wires required to 1920 x 3 + 1080 = 6,840. That's much more reasonable. This nicely maps to the video signal, which arrives one line at a time.
Generally the way the pixels are built on an LCD screen is there is a transistor patterned onto the glass for each pixel, and then each pixel looks like a capacitor. The screen ends up looking like a write-only DRAM. The capacitance of the pixels will hold the level between updates. In an LED display, each pixel also has an LED which will discharge the capacitor between frames, leading to the flickering. A good solution might be just to double the frame rate so that the flickering is at a higher frequency and therefore less noticeable. |
H: What determines a load's current requirements?
Apologies if this question has been asked before. I'm sure it has but hard to find as wording of question may not bring up the duplicate question.
Let us say we have a 10 ohm resistor as a load. Under a power source of 5V, it will draw a 500mA current. In this scenario, we had two "givens": the resistance and the voltage.
Now here is what I am wondering. Does a 10 ohm load think (using the above resistor as a figurative character)
"Ok, I never know how much current I need. For me to know how much current I need, I need to know what the voltage is since I already know my resistance. If your voltage is 5V, then I will take 500mA. If your voltage is 10V, I will need 1A. But don't give me more than 20V because I won't be able to handle that much of a voltage drop and 2A of current!"
So does this mean a load in series will always draw what it needs to comply with Ohm's law rather than preserve itself from taking on too much current? (i.e. taking the above example, "Load sees that the voltage is 30V "Uh oh, I need to take on 3A. This will destroy me but I must draw it anyway to comply with Ohm's law. So long!"?"
AI: Using the water analogy, a 10 mm pipe will only allow a certain amount of water to pass, and it will depend on the applied pressure. If you double the pressure, the flow will double.
A certain material will require a certain voltage to flow a certain current. Quoting a nice answer in Physics.SE about gravitation:
Physics [Ohm's law] does not answer existential problems. It gathers data and observations and models them with mathematical equations and functions, and then can explain the data with the model and predict new observations. |
H: Do I need a base resistor for a BJT?
I have a 74HC595 shift register that I shift bits into from an 5v Arduino. The pins on the shift register is connected to the base of 2N3904 BJT's through a 4.7kOhm resistor. I have noticed that if I include the base resistor, my current to the base is 12.8uA and if I don't the base current is 13.1uA.
Since the figures are so small, do I even need a base resistor?
I should mention that I am trying to build a circuit for one of those fancy LED cubes. On the schematics are shown 2 diodes, but that will be expanded to 8 LEDs for each shift register.
simulate this circuit – Schematic created using CircuitLab
AI: The base resistors R3 and R4 are doing nothing useful. The circuit would be better with them replaced by wires.
You often see this kind of nonsense when someone designs a circuit by heresay and rules of thumb instead of actually undestanding the electronics. Whoever designed this heard somewhere that you're supposed to put a resistor in series with the base, but didn't bother to listen to the reason why and when this "rule" is appropriate.
Q1 and Q2 are used as emitter followers, so the base and emitter current will be nicely limited to safe values just because of the impedance of the load on the emitter. Adding base resistors will only make the output voltage less predictable.
Note that Q3 is used as a common emitter amplifier with the emitter tied to ground. In that case, something is needed to limit to the base current to a safe value. That is what R5 is doing.
So in summary, R3 and R4 are not needed and the circuit would be better off without them, but R5 is needed. Again, you have to undestand a circuit, not blindly apply rules of thumb, heresay, or any other type of silly superstition. |
H: Will ARM replace that older processor architecture?
I know that those processors with ARM architecture have their advantages, but will it replace those old kind of ones? Or they just will be dealing with different kinds of applications and continue to live?
Edit: I am not seeking a fotrune teller. I just needed someone who knows it to describe what is going on right now? How does it seem to go?
AI: It takes a long time for an architecture to die.
The 8-bit instruction sets aren't even dead. You can still get Z80s: http://en.wikipedia.org/wiki/Zilog_Z80#Second_sources_and_derivatives , and the 8051 instruction set is also widely available.
The 16-bit instruction sets are still shipping; 68k in the form of Freescale "ColdFire", and 8086 in the form of 16-bit bootloader code that's still present on some PCs that don't boot straight through UEFI. Microsoft have only fairly recently dropped support for 16-bit Windows applications.
32-bit x86 will continue to ship in some form for as long as someone needs to run Win32 applications. That's good for at least another couple of decades. |
H: Can I use the 12V line of a computer PSU to power my circuits?
Or is there any issues I should be wary of? I need at least 7A on a 12V line, but I've found that a computer PSUs are significantly cheaper.
AI: Yes.
There's not much more to say than that ;)
Getting them to turn on though can be a bit tricky. Some may require a load on the 5V before they will allow you to turn them on (a simple resistor will do), some may require the load on the 12V (in which case you're fine), and some don't care either way.
To switch a normal ATX power supply on you need to connect the green wire to ground. I often use an NPN transistor to act as an MCU controlled switch to control the power.
I have a small one embedded in my desk powering all sorts of things - PIC32s, audio amplifiers, a bunch of binding posts for powering whatever I'm working on... It's a really handy power source, The -12V is also great to have available. |
H: How can I generate a sinusoidal voltage and current waveform using half-bridge or full bridge inverter?
I know how a half bridge and full bridge circuit looks like. What I don't understand (and I simulated it) is that whenever I drive the switches using a pulse generator I get a square wave as an output. I am not filtering it.
So my question is: Can I get a sinusoidal voltage waveform and a sinusoidal current waveform using sinusoidal PWM (so dutycycle of PWM varies constantly in order to change the average DC value to create a sinusoidal waveform)?
AI: Start here to get a general idea - Many many ideas here
Leading to
http://forums.parallax.com/showthread.php/130577-PWM-phase-sync
The high:low ratio of the PWM over 1 PWM cycle sets the analog value represented after filtering. The PWM ratio is adjusted accordingly. But how to do so ...
Here's how:
The comparator compares linear ramp & sine wave magnitudes - and toggles the output according to the result.
And the next one is like unto it ...
I expect you to now run from the room crying "Eureka".
YeeHa!!!
This
Yee Ha !!!
From Here
Makes this:
Notes:
Note that one diagram above uses a triangle wave and the other a sawtooth (ramp up, vertical down). Look at what they both do and how the method works. Once you understand what is being done you can "easily enough" convert it into software. The "ramp" in both cases starts at zero and progresses to Vmax over a period of time. With an analog comparator you continually compare the sine wave with the ramp and set the output high when the ramp is lower than the sinewave and low when the ramp is higher. In software the comparison is done at regular time steps
The overall ramp frequency needs to be higher than the sine wave frequency. The more ramps per sinewave cycle the more often the waveform will switch and the higher the PWM frequency and the more losses in MOSFETS or IGBTs etc. But, lower ramps per sinewave lead to a worse approximation of the sine wav when the PWM is filtered. Some systems change the number of samples per sinewave depending on the frequency to limit maximum PWM rate. As sinewave frequency gets higher they will occasionally reduce the ratio. When such systems drive a motor you can hear them "gear changing" a the motor accelerates. Starter from stationary the inverter PWM can be heard as a low frequency tone which rise towards a whistle as the motor speed rises, then suddenly drops by a factor of maybe 4 or 8 or ... and then rises again as the motor continue to accelerate. It may do this several times. This allows the purest possible sine wave at slow speeds and limits drive frequency components at high speeds.
Note that you need model this for only a quarter cycle in software. For the second quadrant you just run the comparison backwards to quadrant one, for Q3 you invert Q1 (highs and lows swapped) and for Q4 you invert Q2. The sinewave values may be calculated on the fly or stored in a table depending on what suits your system. If you have lots of ROM/memory room it may be easier to use a whole 360 degrees in memory but using 1/4 cycle 4 times is not much harder.
So, if using a table:
Q1 - Up table
Q2 - Down table
Q3 - Up table, output inverted
Q4 - Down table output inverted.
Which reminds me of an old old joke*.
What can go:
Up a chimney down and
Down a chimney down
but cannot go
Down a chimney up or
Up a Chimney up
?
(In some case, trying to go up a chimney up may work after a fashion if you try hard enough but may lead to disaster but down a chimney up always ends in disaster. ) |
H: Why are synchronous counters synchronous?
Doesn't the input signal having to propagate through the AND gates cause a propagation delay anyway? With the asynchronous counter the delay was caused by the clock signal - with the synchronous counter wouldn't the delay exist anyway, just with the input instead?
AI: With synchronous counters the delay occurs while the clock is inactive (unchanging, or an inactive edge). When the next active edge arrives at the clock inputs, the data inputs are already stable. Hence all outputs change at the same moment, which is the essence of the system being 'synchronous'. |
H: Possible to use external graphics program to design KiCad edge cuts?
Is it possible to create the outline for your PCB using a drawing package and then apply it within KiCad somehow to make your edge cuts? I am designing a board that must fit into a 72 pin SIMM socket and getting the exact dimensions and shapes for the key holes / registration shapes is proving to be a royal pain in the butt!
AI: So after further search I came across a two+ step solution. It involves importing a monochrome image using Bitmap2Component within kicad and saving it in a mod file following this tutorial then to get correct dimensions run the mod file through the online KiCad Bitmap2Component Scaler.
note that the scaler utility allows you to choose the kicad layer it saves to so you shouldn't need to edit the layer by hand as suggested in the first link.
It's a bit of a kludge but hopefully it gets the job done for others. |
H: Modeling FPGA logic element responses
I'm working on modeling a circuit implemented on an FPGA, and the fundamental question I keep running into is this: what is a logic element? I need to be able to model the temporal response of the output when the input changes as accurately as possible. Right now we are using a simple RC circuit driven by a binary voltage source (corresponding to the implemented logic function) with a time constant of ~400 ps, but clearly that's too simplistic and it's not accurate enough.
Looking at e.g. the Altera handbook, LEs are implemented essentially as a look up table with routing components around it. Can someone point me in the direction of (a) a circuit diagram of one of these look up tables or (b) a more appropriate modeling framework? I'm new to this, so please be gentle.
AI: A Logic Element (LE) is made up of a number of gates. Exactly how long it takes to propagate data from the inputs to the outputs depends just on the combination of gates that the signals pass through, and that depends on how the LE is programmed.
The main controlling factor of the LE is the Look-up Table (LUT). This has 4 data inputs and 4 data outputs, and is basically a little block of static RAM (16 nibbles). The RAM is loaded when the device is programmed by the configuration chip. The incoming data signals are the address lines of the SRAM, and the output data signals are the data bus of the SRAM. A change of address (incoming data) yields a pre-set change of data value (outgoing data).
For modelling the timing you would really need to know which gates are in use for which values of the LUT and factor the propagation of those into your calculation. For the LUT itself, it's just SRAM, so you can just model that as a small piece of SRAM.
The actual layout of a LE in different modes of operation can be found in the document Logic Elements and Logic Array Blocks in Cyclone IV Devices. |
H: Where does the power supply go in an SR Latch?
I understand how an SR Latch works: is S is 1, it will set Q to 1. If R is 1 it will set Q to 0.
If both S and R are 0, the value of Q should remain unchanged (right?). So the circuit has these stable states:
Now my question is: If 0 voltage is flowing through both the input wires, how can the 1 still exist in the circuit? There must be an external power supply? (I'm assuming there must be because, if you disconnect your computer, everything in the RAM gets wiped) but where would this power supply go without disrupting this circuit?
I hope my question was clear. I just don't understand how it can keep its value when the inputs are 0.
AI: Of course there's an external power supply. Take a look at a NOR gate chip (say, 74HC02) and you'll see it there clear as day:
Logic symbols purely show the logic flow of a circuit, not the power flow. Many schematic capture programs will take the power connections as read and automatically connect them to the right nets for you.
Internally a typical CMOS NOR gate is made up of MOSFETs which switch the power coming into the chip in the right way:
You can see there how the output can be switched either to provide power from V+ or to sink power down to GND depending on how the inputs control the MOSFETs. |
H: Best way to make LEDs flash in time with a music source
Could someone suggest the best circuit to make LEDs flash in time with a music source directly connected to the circuit? BTW the circuit needs to run off batteries, I've seen circuits using TIP31 transistors, but are they any good?
AI: There's a website for analog audio circuitry that I really like: https://sound-au.com
In keeping with the author's request, I've only provided a link to the home page, but if you add these to the end of the URL, you'll get:
/project60.htm
LED volume meter
/project136.htm
Real-Time Analyser (RTA), basically a full spectrum of what's happening at the moment
/project62.htm
Complete lighting system, including a "Sound to Light" (S2L) module at the bottom of /project62a.htm
Perhaps you could adapt one of those?
Or if all you want is to flash a single LED in sync with, say, the kick drum, then you could use a dual opamp to:
Lowpass the signal so you're more likely to trigger on the kick rather than the hihat
Compare the result to some reference level
The result of that can either drive the LED directly if the opamp is strong enough, or feed a transistor. |
H: Determining End-Of-Life for electronic components
What is the best way to determine the end-of-life time-line for ICs other than contacting the manufacturer directly? Is there a central registry or service where I can subscribe and get EOL notifications for the parts used in my designs? How does everybody else handle EOL problems?
AI: There are few professional services which integrate notifications from manufacturers and can manage your BOM:
IHS: PCMS, BOM Manager, PCNAlert, etc.
SiliconExpert
Probably others I'm not aware of.
Most of big companies in aerospace and defense business subscribe to one of these services but they have dedicated component management departments.
Also only few components really need attention for EOL notifications, for instance memories (DDR). EOL notifications are submitted almost 1 year in advance so you can check the PCN on a monthly or weekly basis of few manufacturers for your critical components. |
H: Does ferric chloride etchant "go bad" after some period of time?
I have some old circuit board etching solution (ferric chloride) that is unused but a few years old and has not been in a temperature controlled environment. It has been sitting, all but forgotten, on a shelf in my garage.
I'd like to etch some small boards this weekend, but would rather not ruin them with old chemicals.
Should I just go ahead and use it or buy some new etchant?
AI: I have successfully used ferric chloride from a ten year old bottle. It was stored in my house though, not the garage. |
H: Reset I2C slave chip when master goes away
I have a board with a transformer and a MCP23017. This board is connected to another board via 4 wires which bring over power (+5V, GND) and I2C bus for an ATmega328P acting as the master.
The ATmega will enable some pins on the MCP23017 when the user selects an input. The I2C pull-up are on the ATmega side.
Once a pin is enabled the MCP23017 will keep it enabled until reprogrammed or reset.
I want to reset the MCP23017 if the cable is disconnected, in order to switch off the pins, but I don't know how to detect it, given the fact that unplugging the cable is severing the I2C connection (and removing power from the ATmega).
The only solution that comes to my mind is to use a second uC to ping the main ATmega and remove biasing on the MCP23017 if the main board is not answering anymore, but I'd love to find a simpler solution.
AI: Since you say that the I2C-pullups are on the ATmega side maybe you could just run a timer and check if both SDA and SCL are low for longer than expected during normal communication and reset that timer once the signals go high. Maybe not the most power-efficient way but does not need any additional circuitry. |
H: How are radar returns sorted in range space in Synthetic Aperture Radar?
I'm trying to understand how a SAR system is able to place returns it receives in range space.
Consider the scenario when an airplane is passing over terrain and is imaging it using SAR, as shown in this image from radartutorial.eu:
If it relies on the timing of the return, it seems that there would be ambiguous cases where a tall object further in the swath could have returns that are received earlier than a short object in the near swath. How would a SAR system ever sort out the elevation of the terrain being scanned in the swath?
From what I've found it doesn't seem like there is any sort of servo tracking the beam along the swath width (and it would have to be moving quite fast for this to work). This leads me to think that there must be some signal processing wizardy used to sort these returns that prevents timing ambiguities. What is going on?
AI: As you've mentioned, all radar works by measuring the TOA (Time of Arrival) of a return pulse that has reflected back from an object. However, SAR uses a lot of special signal processing to compensate for the fact that the actual antenna is much smaller than the synthetic aperture.
The movement of the physical antenna from the time it emits an incident pulse to the time it receives a reflected pulse is what makes the synthetic aperture. So, among other things, the doppler effect of that movement on the frequency of the pulse and the fact that different frequency components of the incident pulse get reflected back to varying degrees (group delay) are all measured and compensated.
In addition, modern SAR is focused rather than unfocused. That is, similar to lenses in optics, phase adjustments of components of the radar return can be processed to effectively focus the pattern and increase the resolution.
I'm no expert by any means, but those are the basics. If you want to get a little deeper into it (with all kinds of lovely calculus equations), you might enjoy reading this book chapter specifically on that topic. That document is a couple of decades old, but the principles are the same.
If a picture is worth a thousand words, as the old saying goes, http://www.intro2radar.com/ is worth many, many thousands because it has not only pictures but animations. You may find the "ranging imaging" animation particularly helpful to answer your question. It's also worth pointing out that the animations are intended to complement the excellent book, Introduction to Microwave Remote Sensing by Dr. Iain H. Woodhouse. |
H: STMicro library for setting RTC Alarm on Sunday is incorrect?
I'm using a stm32fxx ARM Cortex-M3. When setting an RTC alarm for Sunday, I realized that the stm32l1xx_rtc.h header file defined Sunday as:
#define RTC_Weekday_Sunday ((uint8_t)0x07)
In code, I do this:
rtc_alarm.RTC_AlarmDateWeekDay = RTC_Weekday_Sunday;
But no alarm is fired when I used this value. Instead, I tried inputting 0x0 whenever I want to set an alarm on Sunday, then that seems to work just fine! So here is my hypothesis:
I have used the wrong header file for the F series
I have to do a mod 7 operation when setting weekday? I found nothing on a spec that mentioned this.
STMicro header file has a bug
If someone with knowledge can help me understand this problem?
AI: It is a common situation where the Day-Of-Week is represented as a value of 1 -> 7. You can also find situations where the DOW is represented by numbers 0 -> 6.
I have also seen situations where sometimes Sunday is considered the first day of the week with the lowest DOW number and others where Sunday is considered the last day of the week with the highest numbered DOW number.
You have to do whatever it takes to adapt your application environment to the DOW number system used in your RTC chip. If that takes a little software coding on your part then that just becomes a part of the job.
So I would recommend that it is better to track toward what works instead of trying to figure out what may or may not be wrong with chipset library code. |
H: JTAG Circuit to prevent back powering
Reading a presentation on an MPU evaluation board I found this particular circuit:
Can someone please explain me the logic behind this and what is this "back powering" scenario that the presentations talks about?
AI: Back Powering is what happens when data entering an un-powered device's input pin is routed up through the internal ESD diodes in the device onto its power rail. This often provides enough power to run the device (escpecially with idle-high signals coming in) but with (often dire) consequences.
ESD diodes may burn out
IO pin circuitry may be damaged
The device providing the errant signals may be damaged
It's hard to tell from that circuit, but I have to assume (due to a process of deduction) that it is in the evaluation board, not the JTAG device.
There are three important power circuits in question here - REG_3V3, the power from the internal voltage regulator, VDDIO_3V3, the JTAG IO port 3.3V power signal, and WALL_5V_IN, the main input power to the board.
The op-amp compares those voltages (actually it compares VDDIO_3V3 to 87% of REG_3V3, thanks to the voltage divider R298 and R306). If VDDIO_3V3 is above that 87% reference voltage, then the output of the op-amp is high. That high signal will enable the MOSFETs in the data path to allow the signals through the interface.
However, when there is no power to the board, and when the JTAG is plugged in and powered, VDDIO_3V3 will be at 3.3V, and REG_3V3 will be at 0V, so you would expect the output of the op-amp to be high. But, we're forgetting WALL_5V_IN. As there's no power to the board, that will also be at 0V, so the op-amp will be unpowered. Therefore it doesn't output anything at all, and the voltage divider formed by R297 and R305 between 0V and GND would pull the output into a low state. That then turns off the MOSFETs isolating the JTAG bus from the rest of the board, protecting it from any signals coming in that could damage it.
As for why there's a comparator in there, and not just monitor the incoming power directly by tying it to the gate of the MOSFETs? The user manual explains it as thus:
The signal lines are disconnected when the
i.MX28 shuts down to prevent back powering from the JTAG tool into the i.MX28 processor.
The disconnect circuit is triggered by the falling edge of the VDDIO_3V3 supply via comparator
U48 when the i.MX28 processor shuts down.
The VDDIO_3V3 supply is generated internally inside the i.MX28 controller. It is possible (by the sounds of things) for the board to be powered up (WALL_5V_IN == 5V, REG_3V3 == 3.3V) but for the chip to be shut down and not running (VDDIO_3V3 == 0V), in which case the MOSFETs should be in an off state - hence the comparator to compare the 3.3V generated by the i.MX28 with the externally regulated 3.3V supply. |
H: Real equivalent of terminal resistance
I found in a book a scheme representing a terminal connection used in making an ammeter shunt (with the purpose of extending its measuring domain).
I can't understand what the drawing at b) represents. All those resistances want to model the contact resistances.
Then, the book says that in order to make the measurement more precise, we use a "double terminal", drawn here:
What do these drawings represent in real life and why are the resistances arranged as in the first picture, at c) (the delta connection)?
AI: Drawing b) represents a terminal cut in half. Let's say that the white rectangle where R3 and Rs are connected is a nut, then you have one ring terminal lug between R3 and R2, then another one between R2 and R1, and then you have the head of a screw at the other end of R1.
Here the contact between the screw and the nut is considered perfect (rather surprisingly) so R1 is directly connected to R3 and Rs.
The double terminal allows to eliminate some of the parasitic resistors. On current shunt you often find 2 big terminals for the circuit where you measure the current and 2 smaller terminals where you connect your voltmeter:
Likewise, on the SMD shunt resistors you'll find 4 pins, 2 for the main circuit and 2 for the measurement: |
H: How should I test a coaxial cable?
I'm a ham radio operator, but haven't been very active on HF bands for a while. I have an inverted-V antenna and a long run (approximately 25m) of coax that runs from my upstairs "shack" down the wall, through a conduit that I buried in a trench and up near a tall oak tree in which the antenna is mounted.
Other stations don't seem to be able to hear me very well, and so I am suspecting that the coaxial cable may be waterlogged. Other than the obvious tests with an ohm-meter, how can I test the coax to see if it is still OK?
I don't have an antenna analyzer, but do have an antenna tuner and a dummy load.
AI: You can measure a lot of things (impedance, velocity factor, distance to short-circuit, distance to open-circuit, ...) with a TDR (Time Domain Reflectometer) as shown and explained in multiple tutorials.
For example:
Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter
How to measure coax velocity factor VF and impedance Z
"TDR" or Time Domain Reflectometer, build and use this circuit
Determining Velocity Factor of coaxial cable
Understanding DTF or Distance To Fault, using a TDR
Determining Coax Impedance with a TDR |
H: Can I power the VTX and FC from the same vbat pad on the ESC?
I have the Hobbywing x-rotor 60A 4 in 1 ESC and the TBS Unify pro hv race.
Since neither my FC or ESC has a 10V regulator, I need to power both from the battery voltage. There is however only one battery pad on the ESC; will this be okay?
AI: Yes, you can. Generally, you want to have your VTX and Camera on the same ground (in order to avoid video noise), so this approach will definitely work. There is continuity on pretty much every VBAT pad, as well as ground, so having a single pad or multiple pads don't really matter much.
If you're seeing EMI noise effects, I would still highly recommend running a large capacitor directly on the pads of the ESC. |
H: What is the proper method of tuning PID filters for recent BetaFlight (4.x) on micro-sUAS WITHOUT Blackbox?
I've included my specific platform below, but this question should be applicable to other platforms just as well.
Board:
Matek F411
Platform:
BetaFPV Beta65 Pro 1S (65mm m2m)
I know there are serial logging devices specifically to provide BlackBox for boards without onboard flash, but my concern is that given the very narrow power envelope of these micros that any data collected with an external logging solution would be scaled improperly once the micro is flown without the logger onboard.
AI: OpenLager is one solution.
Tuning with auditory inputs is another (it's usually possible to hear oscillations caused by excessive P or D gains), but this is more challenging. This is made significantly more challenging because often props are poorly balanced, and this can imitate the same sorts of errors.
Unfortunately, trial & error tends to be the best answer to this. As far as general trends, in the sliders most often stock master gains, 0.8-0.9 P/D Ratio, and slightly high P&D Gains (1.1-1.2) will work well on quads with good motor authority (1202.5, 1103 >10,000KV for 1S quads). Thrust_linear = 50 is also something that produces pretty great results.
The harder one is trial & error on filters - quite often you can work iteratively towards the right with the gyro all the way out to 2.0, and the D term filtering will end up somewhere in the 1.4-1.8 range with these PID settings. |
H: Why are most multirotors built with an even number of rotors?
The most common types of multirotors are quadcopters, hexacopters and octocopters (with four, six and eight rotors respectively).
Are there scientific reasons why most drones are built with an even number of rotors?
AI: Yaw authority and ease of mixing is generally the answer - since yaw response isn't necessarily the most linear, being able to have N number of motors contribute to yaw in one direction with the same number in reverse is the simplest motor output mixer configuration.
Tricopters do work and are relatively common among odd-numbered multicopters, but once you've exceeded four rotors, there aren't many meaningful performance improvements possible for the added complexity of requiring an odd number of motor outputs and doing a custom mix for that.
There are some slightly exceptional cases such as a forward puller motor mounted perpendicular to an otherwise conventional even-motor copter frame (see FliteTest MilleniumFalcon), but these aren't really a convention odd-numbered motor copter configuration. |
H: Storing drones during the winter
Up here in Canada, it can get kinda cold during the winter. And just because of where I tend to use my drone, it can be easier for me to store it in an outdoor (dry, but unheated) shelter than to haul it back to the house all the time.
Is it okay for drones (Eachine E520S) to stay outside at like -30°C, or should I continually bring it inside? My guess is that it's the batteries that will suffer the most, so should I just bring them in? (lithium-ion batteries)
AI: I can't speak for your specific drone, but I have experience with electronics and cold weather.
I would definitely store the batteries in the warm, and check the manufacturer's guidelines for the correct storage voltage (for a LiPo I think this is about 30% to 40% charge, or 3.85 volts per cell) It also doesn't hurt to store them in a fireproof container for extra safety.
The main hazard I've found when storing and operating electronics in freezing conditions is the risk of freeze-thaw cycles when the temperature is hovering around 0°C. Once it is cold, it's generally OK.
Additionally, if you do decide to use the aircraft during the winter take care when moving it from a cold, dry environment into a warm one - condensation will form and can cause damage or corrosion. Let it warm up thoroughly before use - if possible, seal it into a watertight bag or box before bringing it in to preserve the dry air around the drone, and wait for it to warm up before opening to minimise condensation. |
H: Are there any alternatives to BetaFlight?
I wonder if there are any alternatives to BetaFlight. If there are any alternatives, what are their advantages and disadvantages?
AI: Among open-source options, Emuflight, INav & Ardupilot (different focus) Butterflight (mostly defunct), Raceflight (mostly defunct). There is still Cleanflight and Baseflight, which are the shared lineage for Betaflight, Emu, Butr, and others.
Of these, EMU has an EKF (Enhanced Kalman Filter) variant of filtering, and also in older releases captures a lot of the 'goodness' of BF 3.5.7 filtering with a few PID adjustments that some pilots prefer for feel - and is now where Project Mockingbird does custom whoop tune development.
FlightOne, especially FalcoX adds a lot of ease of use for OSD configuration, and on most setups the stock tune out of the box or selected from a short list of defaults will fly really well. Require FL1 FC or FL1 license on specific F411 targets (at the moment) - FL1 makes great ESCs if you can get ahold of them.
KISS is also a closed source, with mostly one source of ongoing development, but provides impressively good flight performance as well. Needs KISS FC to work, I do recommend other ESCs.
For closed source options, the biggest downside is cost and need to re-learn some of the approaches if you're familiar with Betaflight already.
The biggest feature-set of Betaflight lies in the RPM Filtering capability (which in my experience can make cheap builds outperform most people's expensive stuff), although for many users without a background that makes the tuning process very accessible, the 3.5.7 builds of BetaFlight will fly better in stock or near-stock tunes. |
H: How do you figure out which motors to use for your UAV?
There are a ton of electric motors on the market. Is there a mathematical formula or some other means of figuring out which size of motor you will need for your airplane, helicopter, or drone?
AI: N.B.: This applies particularly to drones, and to a lesser extent to helicopters. Airplanes are a totally different field and would require a separate answer.
Obviously the more power per weight... the better. But basically you want to be able to get enough energy out of the motor to be able to hover at around half throttle. There's a good tutorial here which explains the full process, but basically you'll have to do the following:
1. Make an estimation of the total weight
Be sure to include the weight of your motors in this calculation.
2. Multiply that weight by 2
This compensates for the fact that the drone has not only to hover but also to move upwards through the air. If you build your drone and find it's too sluggish for what you want, simply increase this factor.
3. Find an engine that will generate 1/n thrust
...where n is the number of engines that you will have on the drone. So for instance, if you calculated that the weight of your drone was going to be 800g, you'll want each motor to have roughly 400g of thrust.
For most propellor motors designed for flight, you'll be able to find what is called a "thrust table" which will list propellor size (be sure your body is big enough to leave space for the propellors to spin independently!), the amount of current it burns, thrust (usually measured in grams), etc.
Do the comparison, and you're set! |
H: Why did a quadcopter in vertical descent appear to lose thrust and descend rapidly?
Towards the end of a flight with a quadcopter, I started a vertical descent to come in and land. There was no wind on that particular day so I just came straight down.
Shortly after starting to descend, the quadcopter started falling faster. I increased the throttle to compensate, but it only seemed to fall faster.
After a rather unceremonious landing, I checked the quad over and found it still had power enough to take off and hover, so I don't think it outright lost power. It came down level with the motors going.
What would have caused the rapid loss of vertical thrust?
AI: It sounds like you've encountered what's called "vortex ring state", when a rotorcraft descends into its own wake -- essentially, a self-created downdraft.
There are two ways to escape from such a situation:
Power out of it. Climbing out of the craft's wake vortex requires about twice as much power as hovering, so while most full-sized helicopters can't do this, most quadcopters and other small-scale craft can.
Move forwards (or backward, or sideways) out of the craft's wake. This is the standard procedure for helicopters.
You can also avoid it entirely by not descending too fast, or by not descending straight down. |
H: Why was 32kHz gyro sampling removed from Betaflight?
For a while, the BetaFlight multirotor firmware supported 32kHz gyroscope polling, but now that feature has been dropped in favor of 8kHz being the highest polling rate. Does anyone know why this was removed?
AI: In order to access the 32kHz gyro sampling mode, the gyroscopes in question (e.g. ICM 20689) would be switched to an experimental mode where there is higher gyro data jitter, and that sampling at the lower 8kHz frequency made use of lower overall latency lowpass filtering built into the gyro.
Often to get usable gyro data from the 32kHz sampling rate, more filtering was required, and the overall phase delay response meant that the FC would not be able to start compensating for a change in craft orientation any faster.
The other part is that in terms of pure filtering performance, the net gain achievable assuming identical gyro signal-to-noise ratio performance is on the order of tens of microseconds. The bigger potential improvement would be to use the higher sampling rate to enable enhanced/fast Kalman filtering where the PID calculations can take into account more data, however, the input noise in that experimental 32kHz mode means the net improvement is minor for the computational cost involved, and that better performance is achievable at the lower sampling rate the IMU units available right now (that meet the cost and form factor requirements to install onto cheap FPV UAS systems)
Practically, lower overall phase delay response with RPM notch filtering and moving lowpass filters well beyond the prop wash and flight input frequency space, which results in much better overall performance, and often reduced motor heat in flight across a wide range of flight envelopes because less amplification of gyro noise is occurring, particularly through the D term of a PID controller which amplifies higher frequency oscillations. |
H: How low can I safely discharge a LIPO battery during flight?
What is the minimum voltage I can safely discharge a LIPO battery during a flight?
For example, if I have a 3S LIPO battery, should I be aiming to land my RC plane before the battery gets below a certain voltage?
Related to that, is it the voltage under load that matters, or the open/resting voltage once it is disconnected?
AI: The short answer is that you don't want to run batteries below 3.0V ever, and ideally want to avoid large current draw below 3.5V. For a 3S battery, once you start seeing voltages in single digits (3.33V/cell is 10V, below that), it's time to land as a quick reference point - although you can set more precise values of around 10.6V as the warning point through telemetry at a transmitter and/or OSD on an FPV display.
The real issue you're trying to avoid here is that in the Lithium-CobaltOxide (LiPO battery chemistry), below 3.0 volts (can be 2.8V or so in Li-Ion batteries), the battery starts plating lithium permanently onto the anode reducing the capacity and discharge performance of the pack. Doing this greatly harms the battery, so avoid that at all costs.
The reason I reference the 3.5V point is that typically you have another 10% of battery capacity left in order to affect a safe landing approach pattern without being under time pressure to land quickly, and make a good safe landing - also under load, the battery that is at 3.5V in flight will tend to recover to 3.7-3.85V depending on craft and setup, which is more appropriate for storing a battery as you travel back home to the bench charge the battery to its proper storage voltage (3.8V). |
H: Why do 48KHz ESCs increase flight time?
I was looking at ESC settings for whoops/toothpicks and have seen how the 48KHz JESC firmware can increase flight time. Why is this? Thanks!
AI: To clarify, the 48kHz is NOT running on the gyroscope or the FC firmware.
What 48kHz is referring to is the PWM Frequency in the ESC, namely the frequency at which the power MOSFETs on the speed controller switch on and off in order to provide power to the brushless motors and therefore thrust to the craft.
The reason in whoops and small/light craft 3" and smaller will see a flight time improvement is that the RPm ranges these crafts fly at, the switching down-time at lower PWM frequencies means the motor/ESC is having very small active braking events in the switching cycle, then overcompensating by driving higher throttle values to compensate (both of which cost energy).
In some cases, 96kHz will also work very well, although this is often limited to smaller whoop class quads, and I would caution against it on specific instances where the ESC design has a higher dead time value because these tend to suffer failures more often at higher PWM frequency when pushed hard. |
H: What does the C rating on a LiPo battery mean?
I see LIPO batteries specify a C rating on the label.
What does that stand for and how does it affect the performance of the battery?
AI: The C rating is a multiplier that dictates discharge. You can figure out the maximum theoretical discharge by multiplying the C rating by the capacity. For example an 80C, 1300mAh LiPo can output 80*1.3 = 104 Amps. There is no consensus as far as I am aware as to what the letter C stands for.
It is worth noting that C rating is dubious - comparing the C rating between different manufacturers is unlikely to produce useful results as to which battery is better. |
H: How does weight factor into legal allowance in Germany?
I'm in Germany and I want to buy a drone. Are there any weight-dependent flight restrictions?
AI: There's quite a good summary of the rules on the website of the federal transport department (BMVI):
https://www.bmvi.de/SharedDocs/DE/Publikationen/LF/flyer-die-neue-drohnen-verordnung.pdf?__blob=publicationFile
It appears, that there are these restrictions:
For everything < 250g no additional restrictions apply. It's still not allowed to fly at certain places (parliament, airports, ...) and you must stay below 100 m and you must always be able to see the drone.
For everything > 250g you need to label your drone with your name and address
For everything > 2kg you need to participate in some governmental training and have a proof of ability
For everything > 5kg you need special permission from the state aviation department (Landesluftfahrtbehörde). |
H: What sort of fuel do model jet plane engines take?
I've seen several gas powered model jet airplanes at air shows. Do these smaller engines take the same type of fuel as their full-size counterparts, or do they just use 2-cycle gas like their propeller brethren?
AI: I think the only real answer is... it depends on the model. Obviously producers can make whatever they like, though I've never actually heard of a model plane which runs on actual jet fuel (probably because it's far more difficult to get your hands on jet fuel than on the fuel you can get at any gas station).
In fact, some models even come with an electric option (Electric Duct Fan). |
H: How can I safely parallel charge my LiPo batteries?
I have multiple LiPo batteries that I want to charge up so I can get out flying as fast as (safely) possible.
With my existing LiPo charger, how can I parallel charge multiple batteries at once?
AI: Only parallel-charge batteries with the same number of cells.
Safe parallel charging requires that the voltages on individual cells be close to each other. A difference of 0.1V per cell is considered to be the maximum safe difference (I found it online, and have used ever since, although I have no scientific proof of that).
One way to compare the packs before parallel-charging is to compare their total voltages and make sure that the maximum difference between any pair of packs does not exceed 0.1V times the number of cells. However, this may be unsafe due to the fact that some packs may have more unbalanced cells than others, so it is better to compare each pair of cells that will be charged in parallel.
With many packs of uneven charge, it may be possible to divide them into groups whose cell voltages are close enough to be safe to parallel-charge and then charge each group separately.
A parallel-charging board with enough protection is recommended, with fuses limiting the total current of each pack being the minimum. Some boards also have fuses or current indicators for each cell of each pack for more safety.
Choose an appropriate charge current, which equals the sum of charge currents of all parallel-charged packs. Charging at 1C times the number of packs seems to be reasonable and safe if there are no large differences in capacities and cell degradation between packs.
Regular safety rules of charging single LiPo packs apply, for example never leaving the batteries unattended while charging, keeping away from flammable materials, etc. |
H: What is a ground loop and how does a common ground reduce video noise?
To reduce video noise, it is often recommended to add a common ground between VTX and camera to avoid ground loops. What is a ground loop, how does it add video noise and how can a common ground negate its effects. Thanks.
AI: If enough current flows and two ground pads are far enough apart, there might be a voltage difference between them because of the resistance in the PCB:s traces. That's called a ground loop.
As for why that causes noise; if the camera and the VTX have different reference points, a signal sent from the camera won't be received correctly by the VTX, and since a PAL or NTSC (The types of video that is used for analog FPV systems) signals use less than one volt, even a small in potential can cause video noise that will appear as horizontal bars moving vertically.
Here is an some more information if you would like to read more about it |
H: Is it safe to fly LiPo batteries after they get "puffy"?
Some of my LiPo batteries got "puffy" after a few months. Is it still safe to fly them, or should I safely dispose of them?
AI: You should dispose of them.
A LiPo battery has three parts: the anode (negative plate), the cathode (positive plate), and electrolyte (sandwiched in between the two plates). Electricity is formed by electrons moving from the cathode to the anode through the electrolyte (which helps the electrons flow).
The electrolyte decomposes over time, which results in the formation of elemental lithium and oxygen. Some of it combines into lithium oxide and sticks to the anode and cathode, but some of the oxygen remains a gas. This gas is what causes the puffiness you see.
Oxygen likes to burn, so if you use a puffy battery, the likelihood that the moving electrons will ignite the oxygen is much higher, so once it becomes puffy, you should stop using it and dispose of it. |
H: Should Crossfire transmitter antenna orientation match receiver antenna orientation?
I've always heard that you should orient the antenna on the Crossfire transmitter vertically, with the ends pointing up and down. I've also heard that you should do the same with the receiver antenna (immortal T), but on many builds that is difficult or less durable, so I mount the antenna horizontally on the drone (under an arm, for example).
For the best signal when flying a drone that has a horizontally mounted Crossfire antenna, should I also rotate the transmitter antenna horizontal to match, or is it better to keep it vertical regardless?
It would be great to understand the logic either way as well. Thanks!
AI: A dipole antenna is linearly polarised, which means it radiates most of its energy in one 'plane' relative to the antenna. For maximum efficiency, the transmit and receive antenna should be orientated such that their plane of radiation is the same, or very close to.
It is important to note that as the aircraft flies around, the orientation of the antenna changes with the aircraft. The reason a vertical orientation is recommended is that aircraft - particularly multirotors - generally1 fly with one side 'up'; therefore, for whichever direction the aircraft is travelling the antenna is still vertical (or, mostly so) compared with the transmitter.
With a horizontally orientated antenna, when the aircraft turns through 90° relative to the transmitter, the antenna is now 90° out of alignment to the transmitter; it may help to think of/visualise this as "North/South" and "East/West", and the loss of signal will be comparable to vertical vs. horizontal as the angle difference is the same. Some receivers have multiple antennas and recommend orientating them at 90° to each other to mitigate this scenario.
What you may consider, if your transmitter permits it, is to have the antenna at a 45° angle as a compromise for signal strength. Most drones have a built-in lost comms failsafe so a loss of signal should not be a huge concern; for a fixed-wing model aircraft, I would be more concerned about matching antenna orientations.
[1] The main exceptions probably being stunt aircraft |
H: How can I reverse the direction of a brushless motor?
I have a brushless motor on my RC plane. After connecting it to an ESC and testing it I've found that it is spinning in the wrong direction.
How can I reverse the direction the prop is spinning?
AI: Swapping any two of the wires connecting a brushless DC motor to its ESC will reverse the direction it spins.
Despite the name BLDC which implies that the motor runs on DC power, each of the three wires (connected to one of the three phases in the motor) is driven by the ESC with a phase-shifted AC waveform. Each wire's waveform is shifted by 120 degrees from the other two.
This offset is what allows the swapping of two wires to reverse the direction of the motor's rotation.
Because nothing special is required to reverse the motor direction, it is also possible to fix this by editing the software configuration, which will do the same thing but in software. e.g. in the BLHELI32SUITE software for ESCs with the BLHELI_32 firmware installed |
H: What does it mean for SBUS to be an inverted protocol?
I've heard talk about how F4-based flight controllers can't natively support the SBUS receiver protocol without special circuitry. What does this mean and what hardware is needed to invert an inverted protocol like SBUS?
AI: A serial signal is composed of a series of High and Low electrical signals sent down a wire. In the standard serial protocol, a high signal is 0 and low is 1. In an inverted signal this is reversed so that a high is 1 and low is 0. You might think it makes more sense that low should always be 0 and high be 1 but electronics engineers decided that standard serial protocol should be the former.
To convert a standard serial signal to an inverted signal you run it through an inverter chip (which can be as simple as a single transistor) which will simply change all high signals to low and vice versa.
F3 and F7 chips have inbuilt inverters on their hardware uarts, which enable them to transmit or receive serial signals in any polarity. For some reason, F4's were designed without that feature and thus do not support inverted signals on their uarts. |
H: What's the advantage of props in VS props out?
I've heard that the quad will handle better if you reverse the propeller direction (props out), but is the difference noticeable? Especially for a relatively new pilot. Should you use props out from the beginning if you aren't used to one or the other yet?
Are there more advantages, and are there any disadvantages?
Lastly; why have props in become the standard if props out seem to be better?
AI: As far I know the only difference is on the whoops. They fly much better on props out.
On 2,5-5 inch quads, it doesn't matter. Props in: our camera is dirty, you can stack easier on the tree. Props out: Your flight controller is dirty and then you damage your prop it's a higher risk that pice of the prop damage your flight controller or battery (it is a much higher risk).
Greatings form DE. |
H: Why do brushless motors have 3 wires compared to 2 wires on a brushed motor?
I'm familiar with regular DC motors, which is what brushed motors appear to be. They have fixed stator magnets, rotor coils connected to the shaft and two brushes that power the coils at the right time via a commutator. By varying the amount of power via the two leads connected to the brushes I can vary the speed the motor spins at (more or less).
Yet brushless motors typically have 3 input wires. I can see an arrangement of coils and magnets still, but why does it need 3 wires and a special ESC to drive it?
AI: TL;DR:
Brushed DC motors have two wires because they require DC current (+VCC and Ground), while brushless motors require more complex driving circuitry which controls the AC current required to drive them.
Despite sometimes looking similar on the outside, the functional principles of brushed DC motors and brushless DC motors (aka. BLDC motors) are quite different.
Brushed Motors
These function by physical commutation, where the rotation of the armature breaks and establishes contact with pads which continue reversing the direction of current flow through the coil. This mechanical process perpetuates the cycle of magnetic attraction and repulsion which turns the motor.
Brushless Motors
BLDCs use three wires which are driven by the ESC with a phase-shifted AC waveform. Each wire's waveform is shifted by 120 degrees from the other two.
This is because BLDC motors are indeed three-phase AC motors, each with usually more than one actual coil per phase. Inside the motor, phases are wired up in ascending order, e.g. 1 2 3 1 2 3 ... |
H: What makes mushroom antennae so good at receiving and transmitting omnidirectionally?
I've seen many references saying that mushroom antennae (like the one shown below) are excellent at receiving and transmitting omnidirectionally. I have them on my drones' video transmitters and one paired with a patch antenna on my FPV goggles.
What makes them so good at this?
AI: Most omnidirectional antennae have two factors that work well for them.
1) They have fairly symmetrical and uniform RF profiles. This means that you are less prone to sudden drop-outs in the video as you change the orientation of the antenna as it is less likely that you will line up well with an area of lower RF intensity.
2) They are often circularly polarised, as the mushroom is. This means that that the correct Rx / TX combination effectively cuts out interference from signals reflected from various obstructions, making for a more clean and reliable reception. |
H: How to know if a place is a no-drone zone in the UK?
I live in the UK and often hear that a lot of places are "no-drone zones" meaning flying a drone there could get you a penalty.
Is there any database for these zones as often signage isn't clear?
AI: There are two considerations on whether you can fly somewhere - airspace restrictions and land restrictions.
Details on the airspace can be acquired from a number of common sources - the CAA has a page with lots of in-depth information here1, but there are a lot of third-party websites and applications which present information in a more accessible format (for example, I personally use Drone Safety Map and the NATS Drone Assist app.) Remember to check for NOTAMS, although many websites and apps provide this functionality too.
Land restrictions are not so neatly centralised, so you will need to manually investigate whether your local authority or playing field owners have any restrictions for flying drones from their land. As drone use has increased in recent years, larger numbers of public spaces are making this information available on websites or signs but in many cases, you will need to find out who is the owner and contact them directly for permission.
[1] Wayback machine link. |
H: How to avoid a drone going out of range?
I have a simple drone (less than 250 grams), I want to fly it as high as possible to take photos with it, how can I make sure it doesn’t go out of range considering it has no altitude indicator? What would happen if it does go out of range, would it just fly off?
AI: The first stage is to make sure you read your local laws about altitude and VLOS restrictions.
Once you know what you can legally do, the first stage is to set up a failsafe. Do this by setting failsafe conditions in your radio and in whatever firmware your quad uses. Choose what you want the quad to do during a failsafe, whether it should drop out of the sky or slowly lower.
You can also install GPS and set the failsafe as a return to home, and depending on your firmware there are different ways of doing this.
Another way to make sure you stay in the range is to set up RSSI or LQ depending on your system. This tells you how strong the signal is so you can monitor it.
You can also get an RF meter to make sure there is no interference where you intend to fly.
Finally, you can use a specialty long-range system like TBS Crossfire or FrSKY R9, as long as it is legal in your region. |
H: New Prebuilt Drone flipping on takeoff
I have a small quadcopter drone, immediately on takeoff it flips upside down, this happens all the time.
What could be a potential problem?
Edit: It is prebuilt and bought from a shop and has never flown
AI: Additionally to the options listed above, looking into whether or not:
You may have a reversed or upside down prop
Motor spinning wrong way
FC board orientation isn’t what it thinks it is |
H: Freshly built quadcopter/drone flips on takeoff
I just finished building my first quadcopter/drone and it flips when I try to take off.
What could be wrong?
What troubleshooting steps should I take to fix it?
AI: In my experience, the causes of this problem are (in order of probability )
One or more propellers are on wrong.
One or more motors are spinning in the wrong direction.
Flight controller orientation is wrong (mounted facing the wrong direction)
Motor order is wrong (ie. physical location of motors 1-4 does not match up with what the flight controller expects )
How to solve :
Issue 1. On standard rotation quads (props in) the Front Left and Back Right props should create thrust when spinning clockwise. Front Right and Back Left counterclockwise. Make sure the props will create a downward thrust when spun in their corresponding directions.
Issue 2. On standard rotation quads you will need to spin up each individual motor (eg in Betaflight configurator) and make sure it is spinning the proper direction. See Issue 1. for which direction that should be. To do this take off any propellers, connect to Betaflight and plug in a battery. Go the Motors tab in Betaflight and read the warning and click the "props are off" slider. Now slightly move up the slider for motor 1-4 one at a time and feel with your hand for proper direction. If you find a motor going the wrong direction make a note of which one it is (there might be more than 1). To change the direction of any motor use BLHeli configurator for BLheliS ESCs and BLheli Suite for BLHeli32 ESCs. Google search this for more details on how to reverse the motor direction. Alternatively, you can swap any two motor wires at the ESC connection to change the direction.
This post has a very detailed explanation and diagrams What process should I use to ensure my motors are spinning the correct direction when using Betaflight
Issue 3: Look on your flight controller for an arrow pointing in a certain direction. It should be pointing to the front of the quad. If it is not then this is probably your issue. You can also confirm this issue by connecting your FC configurator (eg Betaflight) and looking at the setup screen where you should see a 3d model of a quad that moves around when you physically move the quad around. If the movement of 3d model doesn't match how you move the physical quad then you have an alignment issue. To fix you can either physically rotate the FC so that the arrow points forward or you can set a board rotation in the Betaflight configuration page.
Issue 4: To figure out if this is your issue, follow the motor spinning steps in the solution for Issue 2. When spinning up a motor, make sure it's physical location matches the motor map picture in your configurator. In Betaflight the order should be 1: Back Right, 2: Front Right, 3; Back Left, 4: Front Left. If you move the slider for motor 1 and the Front right motor spins, you have a motor order problem. This can happen if using a 4in1 ESC and two of the motor signal wires got swapped or if the ESC has a motor order that doesn't match what Betaflight expects. The easiest way to fix is to change the motor signal wires so that the order is correct. |
H: Take drone on a plane in Europe?
I want to take my small (<250 grams) drone on a plane in Europe? Are there any rules regarding this? Should the drone go in carry-on luggage or in the hold?
AI: I am not a legal expert by any means, so I just want to say that disclaimer.
As far as I am aware, you can check in the drone itself, but due to their volatile nature, most airlines will require you to carry LiPos in your carry-on luggage so you can react quickly in case the worst happens.
As always, check with your specific airline and the laws specific to your country of origin and your destination. |
H: Why do the motors on my drone keep spinning up and max out when I run up the throttle on my radio on the bench?
When I test my drone for the first time and I run the throttle up form my remote with the props off the motors keep spinning even when I bring my throttle back down. Will my quad flip out if I put the props on and try to fly it? Why does this happen? It seems fine when I spin the motors up from the motors tab.
AI: TL;DR;
This is a result of I term windup from the PID controller. It's completely expected and normal with props off, and will be fine when you put props on and go fly! Almost everyone new to RC multirotors asks this question on their first build, so don't feel bad!
Detailed Answer
The detailed answer to this question is related to how multirotors maintain stability in flight. Multirotors use a control system called a PID controller. This is a feedback loop control that takes information from the sensors onboard the flight controller and then executes change via the motors based on how the commanded input is different from what it is sensing. For more information check this white paper from the national instruments lab.
To understand this problem, the component of the PID controller we need to examine is the I constant, often referred to as Ki. Essentially this is based on the integral calculation between the sensed value and the setpoint or target value. What this means is that I get stronger the longer there is a difference between the sensed value and the target value. Hence change over time. What you're seeing as the motors slowly spin-up is the buildup of that Ki value getting stronger and stronger as flight controller is trying harder and harder to execute a change that it can sense, but can't impact. No props means no force to execute change, hence the windup. This is often referred to as I term windup.
Here is an excellent video from Joshua Bardwell on the topic: |
H: What is the analog video input pinout on the DJI FPV goggle?
I want to build an adapter cable for DJI FPV goggles analog video input. What is the correct connector type and the pinout for the analog signals?
AI: There is a project on thingiverse that shows the pin connector for the video input of the DJI goggles.
It's a 4 pin 3.5mm jack with the following connections
sleve - GND
ring 2 - Video
ring 1 - Right audio channel
tip - Left audio channel |
H: What should I keep in mind when picking my first drone?
While I don't know much about drones yet, I've seen people use them to make awesome photographs and videos, and I want to be able to do that too.
Since I've never flown a drone, I'm looking for a drone that I can use to learn how to fly, it should probably also be able to hover. I like seeing the videos of drones that fly very fast or avoid obstacles with seeming ease. I'm not planning on sneaking up on wildlife, so it doesn't have to be very quiet.
I've been wondering if my first drone should include some camera equipment already or not. If I crash and break a camera, that might be an expensive crash. But if I don't include a camera, how am I going to judge whether I've learned to fly a drone in a way that can capture nice pictures/videos?
I've seen drones being sold in places like toy stores, electronica shops, and specialist shops. There are huge differences in price ranges, and of course in what these drones can do.
But even with the above in mind, I have no idea how to pick a good beginner's model from all of them. What do you keep in mind when picking your first drone, to practice flying and photography/filming with?
AI: An important first step to take when getting into drones is learning to fly them. The very first drone I would recommend to someone is a cheap drone that can be used to learn to fly. It can be disheartening to buy an expensive drone only to crash it and have lost hundreds or thousands of dollars which is why learning to fly with a cheap drone first is a good idea so you can master the skills of controlling the drone. Your very first drone should be any drone cheap enough to not feel bad when it crashes. The Eachine e010 is a basic drone you could learn to fly with.
If you are looking to get into making videos with drones there are two major types to consider: Freestyle/acro drones or professional video drones. The videos you see of drones going really fast, avoiding obstacles, and doing stunts are high-performance freestyle or racing drones. Videos of scenes and landscapes are often from drones designed to carry a professional camera and are used to get extremely steady, smooth videos.
If you want a relatively inexpensive freestyle drone I recommend the iflight Nazgul5 or iflight Cidora SL5-E because it is a quality drone that performs very well for a very reasonable price. This drone has a camera on it for FPV but a GoPro would be needed to make good quality footage.
A good inexpensive camera drone that comes with a stabilized, quality camera is the DJI Mavic mini. It is not too expensive and it is light enough to not need registration. After that, there are many more good camera drones that are more expensive. |
H: What is the advantage of SBUS over IBUS?
Flight controllers have to implement an inverter to process the SBUS signal. Why is SBUS not just uninverted?
AI: SBUS and IBUS are both digital protocols and for a digital protocol, you define two voltage values to represent an active state and one inactive state. This is done for every digital protocol.
You can specify that 0V is inactive and 1V is active (this is called active high). But you can also define that 0V is active and 1V is inactive (active low).
Many bus protocols are active low and SBUS is also such a protocol. The reason for this choice is that active low signals can handle noise better. This is probably the reason why Futaba designed SBUS this way.
The ARM chips used on the flight controllers have the hardware to decode serial signals (both SUBS and IBUS are serial protocols). This decoder also is built for active high or active low signals. If you want to decode an active low signal (SUBS) with a decoder that is built for active high you have to add an inverter between so that both "talk the same language". This is the case for the F4 chips since ARM did not include an inverter on the chip. For F3/F7 this problem does not exist since ARM included the inverter in the chip and the software can enable/disable the inverter.
You get similar performance from SBUS and IBUS since both are a serial protocol with comparable frame times. |
H: What are 'Modes' of a transmitter / controller?
I have heard a lot of reference to the 'Modes' of a transmitter/controller.
What is the Mode of a remote controller, and what is the difference between them?
AI: The Mode of a transmitter refers to which stick on the transmitter controls which movement on a drone. There are 4 modes (named Mode 1 to 4) but the most common are Mode 1 and Mode 2.
The image below shows which control is associated with which aircraft axis, for each mode. The black text refers to fixed-wing aircraft and the blue text is the multirotor equivalent.
Mode 1 and 2 are the most common flight modes used. Most multirotor pilots fly mode 2 (although there are exceptions, one of the OG FPV pilots FinalGlideAus notably flew mode 1). Almost all "toy" multirotors are Mode 2. In fixed-wing RC mode 2 is very common in the US, while Mode 1 is popular elsewhere.
When starting out, find out what mode the people you're likely to fly with use. Having a more knowledgeable pilot being able to fly your drone, or being able to watch their fingers and see how they're doing a particular trick, or for them to be able to coach you easily is enormously beneficial. Once you're comfortable, if you want to explore the other modes, and see if something else feels better, then go for it. If you don't plan to fly with anyone else and have no strong opinion, go mode 2. Virtually all of the stick cam tutorials you'll watch online are mode 2. |
H: What aviation regulations govern the use of drones in Ireland?
I am thinking about starting to fly drones in Ireland. What are the rules that are going to apply, and where can I find the latest information about the laws that govern the hobby in Ireland.
AI: The use of drones in Ireland is covered by the regulations in the Small Unmanned Aircraft (Drones) and Rockets Order, 2015 (SI 563 of 2015). There are also Aeronautical Notices that apply to the use of drones only. These notices can be checked on the website of the Irish Aviation Authority.
Broadly, the regulations require that any drone flown in Ireland may not be operated:
if it will be a hazard to another aircraft in flight
over an assembly of 12 persons or more
farther than 300m from the person operating the drone
within 30m of any person, vessel or structure not under the control of the person operating the drone
within 5km of an aerodrome
in a "negligent or reckless manner" so as to "endanger life or property of others"
more than 400ft (120m) above ground level
over urban areas
in civil or military-controlled airspace
in restricted areas (e.g. military installations, prisons, etc.)
unless the person operating the drone has permission from the landowner for take-off and landing.
Note that the situation in Ireland will change later this year (2020), when the new European Regulations on the rules and procedures for the operation of drones (or "Unmanned Aircraft Systems (UAS)”) come into force.
The situation is explained in Aeronatical Notice U.06, with the catchy title: The pending introduction of (EU) Regulations in the area of Unmanned Aircraft Systems (UAS) in Ireland, available to read or download from the website of the Irish Aviation Authority.
The most up-to-date information can always be obtained from the website of the Irish Aviation Authority. |
H: Drones to fly indoors
Due to the current situation, I can't go outside to fly my drone. What's a small and easy to control nano drone I can fly indoors (no camera necessary)? It should be easy to control as I don't want to crash into anything but don't want to give up drone flying altogether.
AI: This depends - do you want to fly FPV or LOS, angle or acro?
For the LOS angle, I absolutely love the Hubsan X4 H107C. It’s relatively cheap, flies smoothly and is really robust. There is also a version without a camera called the H107L.
If you want to fly FPV acro (or angle), the Tinyhawk, Mobula 6 and Meteor 65 (though you may need a throttle cap) are all said to be very good. |
H: What is the difference between FPV or LOS, angle or acro drones?
What is the difference between FPV or LOS, angle or acro drones? How are they different?
AI: So FPV and LOS are ways to fly the drone, with FPV being First Person View and LOS being Line Of Sight.
In FPV, one generally has a fixed camera on the aircraft and controls the aircraft’s movements based on what they see on the screen/goggles. This is analogous to VR, however not exactly the same.
LOS, however, means watching the aircraft from a third-person perspective, for example how you would watch a bird fly.
Both of these methods have their advantages and disadvantages, however many people in the hobby find FPV more exciting (though not everyone).
Angle and Acro, on the other hand, are the flight modes.
In Angle, when the sticks return to zero deflection, the quad will self-level, so the entirety of its thrust is vectored down - there will be no acceleration in any given direction (part from vertically based on the throttle). In Angle mode, the drone is not able to tilt beyond a caretaking angle, so flipping, rolling, etc is not allowed.
However, in Acro, the drone has no angle limits so it can roll and even hang upside down with sufficient momentum. The drone will also maintain the same attitude once the sticks return to zero deflection, so needs to be manually leveled.
There is a combination of these two modes called Horizon, in which the drone behaves as if it is in Angle mode during normal flight, however it is still able to flip and roll. |
H: What are the exact specifications of a RushFPV SMA connector?
I am designing a mount for a RushFPV SMA connector and have not been able to find the technical details.
I need:
The diameter of the connector
The distance between the mounting screw holes
The dimensions of the mounting plate in which the holes are cut.
If anyone has this information, I would be very grateful.
Thanks.
AI: The Rush cable is slightly non-standard as it lacks the threads on the holes that the TBS connectors have. It works fine though. The SMA is a standard SMA thread, so the normal hole size for SMA connectors works well. In my designs I usually use a 6.5mm hole for SMA, though some of that depends on the tolerances you're dealing with on your machining. The hole spacing for the small mounting holes is 11.5 mm from center to center. I usually use a 1.5mm hole for the bolt for the Rush SMA mounts, as you can thread an M2 bolt right into the carbon without needing an additional nut, but if you're wanting to use a nut, you'll want to increase that to a 2mm hole. |
H: How can I configure a servo in Betaflight?
I’m looking to make a drone capable of deploying a parachute, so I need a servo release mechanism.
How can I configure a servo and map it to a switch in Betaflight?
Thanks.
AI: There are multiple ways to enable servos in BetaFlight, but since you need only one servo it is recommended to use SERVO_TILT.
For servo to work you need a free pin on your board and this pin must have timer function.
Go to Configuration tab in Betaflight Configurator and enable SERVO_TILT and hit "Save and Reboot"
Then setup the switch you want to use on your transmitter and in Betaflight. Let's say it was AUX4. Make sure it shows in the Receiver tab and that the signal is received.
Then go to the CLI tab and type resource (resource list for older BF) and hit Enter.
What you're looking for is resources with MOTOR or PWM function. For example on my board there are 6 MOTOR outputs. I assume you fly 4-motors copter and you also have some spare MOTOR pads.
You need to remember the MCU pin and MOTOR number and of an output you want to use (C09 and MOTOR 5 for me).
Then you need to type:
resource MOTOR 5 NONE
resource SERVO 1 C09
save
Use your own MOTOR and MCU pin number instead of 5 and C09!
This will remap MOTOR5 pad to be able to work with PWM Servos.
Servos numbering in CLI starts from 1
After reboot you can go to the CLI tab and type resource again to make sure everything worked as expected.
Go to the Servos tab in Configurator and hit the checkbox with with your AUX channel on Servo 0 row (AUX4 for me). If you can't see the servos tab check "Enable Expert Mode" on the top right.
Save.
Servo numbering on the Servos page starts from 0. So Servo 1 from CLI tab is Servo 0 here.
You can now power on transmitter and enable the live mode on the Servos tab.
Go to the Motors tab (remove props if you have LiPo connected!). There are servos output there.
If you switch the switch you set to AUX channel you should see Servo 1 output to change the value.
Now you've remapped MOTOR 5 pad to act as a servo output. You can connect your servo's signal wire to it and find any spare +5V and ground pads for it.
If the servo moves automatically as the quadcopter moves check that the CAMSTAB mode is disabled. The screenshot below shows it active, which you won't want if trying to manually control the servo.
If you don't have any spare MOTOR or PWM pad you can remap LED_STRIP 1 resource if you're not using it for LED.
Here's the guide how you can do this: https://youtu.be/jiD6aC0l1VQ
If you need more than 2 servos to control check Sources for how to use Channel Forwarding instead.
Sources:
1: https://github.com/betaflight/betaflight/wiki/Servos-&-SERVO_TILT-for-3.1
2: https://github.com/betaflight/betaflight/wiki/CHANNEL_FORWARDING-for-3.1
3: https://youtu.be/jiD6aC0l1VQ
4: https://youtu.be/cHi1fKP1LRI
5: https://oscarliang.com/betaflight-resource-remapping/ |
H: Why do higher voltage batteries need lower KV motors?
I've noticed that higher voltage batteries need lower KV motors, why is this?
AI: They don't necessarily, however a given amount of power will be required to rotate a motor at a certain RPM with a given propeller.
kV, the voltage constant of a motor, is the maximum RPM it can turn at per volt of power supplied. Example:
A 4S (15V to make math easy) battery will turn a 2000kV motor at 30,000RPM (with no load, inefficiencies, etc)
a 6S (20V) battery will need a 1500kV motor to achieve the same RPM
In theory, the result is the same amount of power (watts) required to achieve the same RPM/power output/etc. A 4S 2000kV motor will draw say 20 amps, or 300 watts. To get the same power output, a 6S 1500kV motor will draw 15 amps (still 300 watts), and spin at the same RPM. Higher voltage but lower current generally results in less heat, because of ohm's law. Current creates energy loss over resistances (ESC components have internal (small) resistance, which is why they heat up, which is why they have a current rating in the first place).
A lot of Bardwell's videos go into the details of 4S vs 6S, but they work out to be pretty much the same. The reason larger models go with higher voltages (and lower kV to achieve the desired RPM) is because they are much higher wattage, and achieving that higher wattage strictly with higher current is unrealistic (Large scale planes would be drawing 500-600 amps at 3-4S voltages, dangerous and expensive).
To reference an earlier comment, the voltage "rating" of the motor is just the suggested voltage. The actual rating of the enamel coated wires would be much higher, but you will destroy components from drawing too much power under load if you went up to those voltages. |
H: Alternatives to LiPo batteries
LiPo is often the first choice of battery due to its high discharge rate and high specific energy (100 - 265 Wh/kg), defined as its energy per unit mass.
In terms of these two characteristics, what type of batteries are next best alternatives to LiPo?
AI: Many people choose to use Lithium Ion batteries on long range builds.
They have a very large capacity and I have seen them give in excess of 20 minutes of flight on a quadcopter.
The reason that they aren’t often used for other purposes is that their current output is less than LiPo batteries.
This means that whilst they are good for long range flight, they lack the power for aerobatics and racing. |
H: How can I take aerial photography without leaving my house?
I have found a place I can fly my drone, and want to take some photos of it. At the moment it's ill-advised to leave one's home unless it's necessary, and aerial photography of abandoned places is not necessary, so I want to try to do this by releasing piloting my drone from the doorstep.
Assume that this place is within battery and radio distance of my house, and can be reached by a public footpath that joins a road that reaches my house. How can I get the drone there and back with minimal inconvenience (and risk) to myself and others, and under what conditions?
(Creative solutions are encouraged; e.g., some safe way to get a drone to count as a vehicle suitable for road travel.)
AI: When flying in the UK you need to keep your aircraft within 500m horizontally of yourself, below 400ft above ground level and within visual line of sight. If you are able to satisfy these conditions, then you should be able to safely fly the aircraft from your home to the location, take the pictures, and return home.
If, however, you cannot meet these criteria then unfortunately you are unable to complete this flight safely during the lockdown.
Information on UK flying regulations, including distance rules, are available from the Drone Safe website - this is well worth a read, especially the "Drone code" page, and knowing this information will help when/if registering as a drone user.
You should also give thought to what you would do if you had an in-flight emergency - if your drone crashes or you have to land away from home, will you be able to recover it?
Edit: You mention in a comment to this question that you live in an apartment building; this complicates things a little more, specifically due to CAP 393 Article 95 which states
95.—(1) The SUA operator must not cause or permit a small unmanned surveillance aircraft to be flown in any of the circumstances described in paragraph (2), and the remote pilot of a small unmanned surveillance aircraft must not fly it in any of those circumstances, except in accordance with a permission issued by the CAA.
(2) The circumstances referred to in paragraph (1) are—
(a) over or within 150 metres of any congested area;
(b) over or within 150 metres of an organised open-air assembly of more than 1,000 persons;
(c) within 50 metres of any vessel, vehicle or structure which is not under the control of the SUA operator or the remote pilot of the aircraft; or
(d) subject to paragraphs (3) and (4), within 50 metres of any person.
(3) Subject to paragraph (4), during take-off or landing, a small unmanned surveillance aircraft must not be flown within 30 metres of any person.
(4) Paragraphs (2)(d) and (3) do not apply to the remote pilot of the small unmanned surveillance aircraft or a person under the control of the remote pilot of the aircraft.
(5)In this article, “a small unmanned surveillance aircraft” means a small unmanned aircraft which is equipped to undertake any form of surveillance or data acquisition. (b)
(Emphesis mine)
Therefore, unless the building is under your control (as a house might be) you should keep 50m away. What exactly constitutes a building under your control might be another question... |
H: Remap not yet existing uart
I have a flight controller that has one uart to little. So i tried to remap SERIAL_TX 5 (That doesn't exitst on the FC) to pin A09, that previously had SERIAL_TX 1, but that doesn't seem to work. I also tried to remap SERIAL_TX 5 to LED_STRIP, but that didn't work either. Is there something else you have to do to get it working?
The commands I used was:
resource SERIAL_TX 1 NONE
resource SERIAL_TX 5 A09
AI: Serial ports are hardware based, and cannot be remapped. You can however remap software serial ports. Keep in mind not every pin supports softserial either, as an available hardware timer is required. Typically you can use:
UART TX & RX pins
PPM Input pins
LED pins
Motor PWM output pins
If you have one of those free, proceed.
Enable softserial from the Betaflight Configurator "Configuration" tab, then click "Save and Reboot".
Go to the "CLI" tab and type "resource SERIAL_TX 11 A09", hit enter, then type "save" and hit enter.
Go to "Ports" tab and assign the feature you want to "SOFTSERIAL1".
Keep in mind that software serial is limited in terms of how fast it communicate information and the load it puts on the CPU. If you have a very intensive low-latency connection, typically try to avoid use softserial. Softserial is best suited to things like VTX control (smart audio and tramp protocol, etc), GPS, and smart-port telemetry, where latency, speed, or loss of signal during flight isn't an issue. While it can be used for other more system critical links like RC control, I would generally recommend against it, though it is theoretically possible.
Sources:
https://oscarliang.com/betaflight-soft-serial/
http://www.multirotorguide.com/guide/guide-how-to-set-up-the-softserial-uart/ |
H: How important is the plastic coating around a coax?
I was swapping over my pigtails and part of the plastic coating around the coax came off.
The coax is still firmly soldered to the SMA connector - is this salvageable with some heat shrink, or do I need a new coax?
Thanks.
AI: The outermost plastic coating is not too critical; this primarily protects the braid from damage or exposure to elements. For the length involved in your image, you should be fine just adding heatshrink to cover that - and if you extend it over the pin you can get some strain relief too. |
H: What is the meaning of 'control of a building' in the UK Air Navigation Order on Unmanned Aircraft?
The UK drone regulations include the requirement that camera drones are not flown "within 50 metres of any vessel, vehicle or structure which is not under the control of the SUA1 operator or the remote pilot of the aircraft" (source: CAP 393 Article 95(2)(c)) but don't go into detail about what 'control' means.
For the purposes of meeting this regulation, what criteria should be met for a vessel/vehicle/structure to be considered as being under the control of the remote pilot? For example, I would imagine your house would be considered under your control, but (inspired by this question) what would you need to do to consider a shared residence to be so?
[1] The CAP393 document defines SUA as "Small Unmanned Aircraft"; this is analogous to a drone or UAV.
AI: I found what looks to be a reasonable answer in CAP 722 - the UK's drone rules guidance document.
In short, it appears a vessel, vehicle or structure can be under your control if you have the permission of the captain/driver/owner, and any people are under your control. For people to be under your control, they must be willing and able to follow guidance and safety instructions of the pilot.
The long version (correct April 2020):
Article 95(2)(c) makes reference vessels, vehicles and structures being under the control of a remote pilot or operator. A vessel or vehicle could be said to be ‘under the control’ of a person if:
That vehicle or vessel is present for the purpose of participating in the flight operation; and
The operator of the vehicle or vessel (for example the driver of a car or captain of a ship) and any passengers are under the control of the remote pilot and can reasonably be expected to follow directions and safety precautions to avoid unplanned interactions with the small unmanned aircraft; and
The owner or other person with an interest in the vessel or vehicle (such as a lessee) has granted permission for a UAS to operate within 50 metres of that vehicle or vessel.
A structure could be said to be ‘under the control’ of a person if:
The owner or other person with an interest in the structure (such as a lessee) has granted permission for a UAS to operate within 50 metres of that structure; and
Any occupants of the structure are under the control of the remote pilot.
For completeness, as the document referes to control of people:
Persons under the control of the SUA operator or remote pilot can generally be defined as:
Persons solely present for the purpose of participating in the flight operation.
Persons under the control of the event or site manager who can reasonably be expected to follow directions and safety precautions to avoid unplanned interactions with the small unmanned aircraft. Such persons could include building-site or other industrial workers, film and TV production staff and any other pre-briefed, nominated individuals with an essential task to perform in relation to the event.
Spectators or other persons gathered for sports or other mass public events that have not been specifically established for the purpose of the flying operation are not regarded as being ‘under the control’ of the of the SUA operator or remote pilot.
In principle, persons under the control of the SUA operator or remote pilot at a mass public event must be able to:
elect to participate or not to participate with the small unmanned aircraft flight operations;
understand the risk posed to them inherent in the small unmanned aircraft flight operations;
have reasonable safeguards instituted for them by the site manager and SUA operator during the period of any flight operations;
not have restrictions placed on their engagement with the purpose of the event or activity for which they are present if they do not elect to participate with the small unmanned aircraft operation.
Note: As an example, it is not sufficient for persons at a public event to have been informed of the operations of the small unmanned aircraft via such means as public address systems, website publishing, e-mail, text and electronic or other means of ticketing, etc. without being also able to satisfy the points above. Permissions have, however, occasionally been granted for small unmanned aircraft flights at public events where these involved a segregated take-off site within the main event, with the aircraft operating only vertically within strict lateral limits that keep it directly overhead the take-off site. Such flights may also be limited by a height restriction and the tolerance of the aircraft to wind effects and battery endurance. |
H: Is there an international competition that yearly chooses the best drones in the world?
Is there an international competition that chooses the best drones in the world?
Is there a yearly “Oscar” for drones, somewhere?
If so what are various categories that are listed?
AI: I don't think there is any international competition for "Best Drone". The closest thing to that would be recommendations that experienced pilots make and it would depend on what that person liked.
I imagine that if there was an international "Best drone" competition it would have categories such as fastest racing drone, best freestyle drone, best photography drone, most popular drone, etc. |
H: Why does the video cut out on HX-100 drone?
I have a BetaFPV HX-100 drone which I recently got. When I tried to fly it FPV the video feed would cut out and leave a grey screen. The OSD still showed up in the goggles so I'm not sure if there is a problem with the camera or video transmitter.
The VTX was hot to the touch and the camera was warm. It was not the output power setting of the VTX because it cuts out even at a close distance to my goggles. I would appreciate any ideas on how to fix this issue.
This happens when flying normally or even hovering and the grey screen is a solid grey, not static. I have not had any big crashes with it.
AI: This sounds like a loose video wire from your camera to your flight controller. Since you still see OSD, but a gray screen, it indicates that the connection from the flight controller to the VTX is good (that's why you are seeing OSD), but the gray indicates that something is wrong on the camera end.
A warm camera is a little alarming. A warm VTX is normal.
I would check all of your connections to your camera. If they all seem fine, then the problem could be the camera (the warm camera seems culprit to me). |
H: How do I set up LUA scripts on my OpenTX radio?
I’m looking at how to use LUA scripts and was wondering how to configure them I am OpenTX radio.
Thanks.
More info: at present, I don’t want to write my own scripts, only run scripts that I can find online, such as VTX control.
AI: Download LUA scripts you need, for VTX control in Betaflight its: https://github.com/betaflight/betaflight-tx-lua-scripts/releases
Unzip it, you will have folders BF, SCRIPTS, SOUNDS
You need to copy these folders to the SD card root directory.
To do this put you TX to bootloader mode and connect a USB cable to it and access the SD card
Or you can just take SD card out of TX and use card reader to connect it to the PC
If prompted during the install replace/merge existing on SD card files.
Now put the SD Card back or disconnect the USD cable and power up the TX.
You need to go to SD Card page on OpenTX now. Depending on the version and the TX itself it can be in different places.
For example I have QX7 and OpenTX 2.3.5, I need to hold down MENU button and then hit PAGE once.
Go to BF folder and execute (by long pressing) bf.lua. Now use PAGE button to navigate to VTX page.
Additionally if you don't want to go to SD Card each time you can setup telemetry screen:
Go to the Telemetry page of your model.
Find a free screen (Screen 1, 2, etc.)
Select "Script" type for it
Then on the next column you select which script to display, select bf
Now you can run this LUA script from the Home page of the model.
Press "PAGE" button till the telemetry show up and then use "PAGE" to scroll to the screen you selected, like Screen 2, etc.
The only thing is that if you run scripts from TELEMETRY screen, they will use MENU button to skip pages inside the script, instead of PAGE. Because PAGE flips pages between telemetry screens.
If you have error "Script syntax error: not enough memory" you probably didn't check "luac" during the OpenTX install
Sources:
https://oscarliang.com/betaflight-change-pid-vtx-settings-taranis/
https://oscarliang.com/vtx-control/
https://oscarliang.com/t16-lua-script/ |
H: How do I safely dispose of my LiPo batteries?
I have an old LiPo battery that is nearing the end of its life, how can I dispose of it safely?
AI: First is to discharge the battery to zero volts which can be done by using a LiPo charger or a light bulb. It can then be thrown into a recycle bin, but check with your municipality before doing so as some will not take them. There may be a local or regional centre where they can be dropped off.
It is not advised to use salt water as it takes a long time and its corrosive nature to metals can lead to incomplete discharge. |
H: How much does mounting an antenna near a carbon fiber frame degrade signal reception?
By how much is signal reception degraded if I mount my RC receiver antenna next to the carbon fiber frame of my quadcopter?
I've heard that it's a major sin to do this because there is some sort of interference generated by the conductivity of the carbon fiber. Is this true?
AI: This is a huge question, which is why a complete answer is not really possible without specifics on your drone frame, antennas, antenna placement etc.
Long story short - Carbon Fibre is pretty much opaque to 2.4GHz radio waves. It's not interference, as such, it just blocks it, which is why guidance is always given to ensure your antenna placement is such that the frame is not between them and you in normal flight.
For a camera platform drone, like an Inspire, you can be confident of the orientation of the drone so the antennas will always have line of sight to you, but for a racing drone you need to make decisions based on "least obscured" placement, as well as robustness, which is why they are typically in the plane of the frame (but not attached along frame members) or well above the frame.
Pic from getfpv.com showing antenna in the plane of the frame:
And a pic of one of mine with a mushroom antenna raised above the frame: |
H: Quadcopter getting hot after a flight
I have a small quadcopter and after around a 10 minute flight, I can feel it gets very hot. Is this normal?
AI: "Hot quadcopter" is a bit broad of a term, a lot of things can be hot in there.
Maybe it's the VTX, then it's normal, they can get hot up to 80C. You might want to use a heatsink though, depends on your VTX.
It might be the Flight Controller's CPU itself. For example, F7 processors can get hot up to 70C
It also might be motors. If you can barely touch them then:
You might have a bad tuning on your quad:
High D gain
Not enough filtering
Conductivity problem, you might have too long motor screws and they are touching the windings.
Sources and additional info on hot motors:
https://oscarliang.com/check-motor-screws-touching-winding/
https://oscarliang.com/mini-quad-motors-overheat/
https://youtu.be/eo2sjTNS0pc |
H: Why is it bad to power up a drone without an antenna on the VTX?
I've heard a few times now that if you power up your drone you have to make sure that you have an antenna on the VTX. Is that true? If so, what will happen if I power up my drone without an antenna on the VTX?
AI: Short answer: Yes, this is very much true. The VTX sends out radio-frequency (RF) energy through the antenna connector and pigtail, and if nothing (or the wrong thing) is connected at the other end, all the energy gets reflected back and could damage your VTX (or at least will heat it up dramatically).
Long answer:
At high frequencies a thing called "impedance matching" becomes very important.
Basically, when a radio wave travels through a wire, the wire presents a certain impedance (a more "generalized" form of resistance) to the signal. Whenever the travelling wave encounters a change in this impedance as it goes through the cable, connectors and other hardware (for example, it was 50 ohms before a certain connector and becomes 75 ohms after), part of the wave gets reflected back at that point, and the bigger the mismatch (expressed as a ratio*, which in the example above is 3:2 = 1.5), the bigger the percentage that gets reflected.
But as long as the ratio stays equal to 1, the reflection percentage is zero, and 100% of the signal will reach the destination. This is a perfect match: every portion of the path that the signal must take has exactly the same impedance as the others, thus the whole signal path has the same impedance**.
In the real world nothing is perfectly perfect, but it's easy to get close enough that the slight mismatches don't create any issues.
Any properly designed antenna will have about the same impedance (in the frequency range that it was designed for; for a video antenna that would be 5.7-5.9 GHz) as the coaxial cable connected to it, thus achieving a close-to-perfect match,
so (almost) nothing gets reflected towards the VTX, and the rest of the energy gets happily radiated out in the form of radio waves.
However, If there is nothing at the end of your pigtail, the mismatch is as big as it can possibly be, since nothing connected means infinite resistance and a ratio of ∞:1. Thus all of the power that the VTX emits gets reflected back towards it and has nowhere to go other than to heat up the VTX itself. If the VTX is poorly designed, the reflected power could even burn some of the components in it!
Obviously, this gets more important with the more powerful VTXes. If you've got a 25 mW or even a 200 mW VTX, it's probably not going to damage itself if left without an antenna for a while (although you better not risk it anyway), it'll just get very hot. An 800+ mW VTX could do much more damage; in the best case it will just get extremely hot and burn your hands if you touch it. More likely it'll also burn itself and maybe some other nearby electronics if left on for a while.
If you really need to run without an antenna for some reason (which is rare, but possible if you're testing something that needs the VTX to be powered up, but don't want to actually transmit anything), there is an option to terminate the transmission line with a "dummy load" instead of an antenna. The dummy load is just a powerful 50-ohm resistor that is impedance-matched to the VTX and connecting cable's 50 ohms, so it eats all the RF power coming from the cable without reflecting any back. The power gets converted into heat (that's what resistors do) which then escapes into the air via a big radiator glued to the resistor. so the dummy load itself will heat up (the power still has to go somewhere), but nothing else will.
Note that shorting the connector is also going to reflect all the power back to the VTX. 1:0 is the same as ∞:1, so zero resistance is no better than infinite resistance.
Also, beware of antennae that are designed for other frequency ranges. For example, a 2.4 GHz antenna from your router or radio control might look like it's compatible and will probably even have the same (SMA or RP-SMA) connector, so it'll fit mechanically just fine, but at 5.8 GHz it will have a drastically different impedance and thus transmit only a small portion of the power it receives, with the rest being – you guessed it! – reflected back and heating up your VTX.
Notes:
(*): for the purpose of determining the amount of reflection it doesn't matter which way the signal is going, so 50 -> 75 (2:3 = 0.66) is equivalent to 75 -> 50 (3:2 = 1.5).
(**): The most popular value for that common impedance is 50 ohms, so you'll see lots of 50-ohm coax cable, 50-ohm connectors, 50-ohm antennae, etc. There are other standards; TV traditionally uses 75-ohm cables and hardware, and some systems use other values. |
H: How long does a LiPo battery last and how do I know if it needs replacing?
I have an old LiPo battery, how long is a LiPo battery usually supposed to last? How will one know if it needs to be replaced?
AI: This question is difficult to answer without a few specifics.
By ‘how long does a LiPo last’, if you mean when it’s on a quad, then usually around five minutes on a five inch quad.
If you mean how long before it degrades, then that depends on a lot of things:
How low do you run your batteries during flight? Lowering them to below around 3.3V can damage them and reduce the lifespan.
How quickly do you get your batteries to a storage charge? Leaving LiPos at anything other than 3.7-3.8V for extended periods can lead to damaging them.
Do you balance charge them? Some chargers have the option to either charge or balance charge LiPos, so it’s good to check.
Do you charge up your LiPos to 4.2V? Some racers charge regular LiPo batteries (not LiHV as far as I’m aware) to 4.35V, which gives higher short term performance but degraded the battery.
Over all, there are many aspects towards the lifespan of a LiPo, and in general if you treat them right, they will last longer.
Some signs that you may want to retire a battery are visible puffing, if you feel there is a reduction in performance, and if the internal resistance of the cells rises significantly from when they were new. |
H: USB Webcam Parts for FPV?
I have a camera taken out of an old usb webcam. It has four wires attached to and labeled on the board: 5V (red); DM (white); DP (green); and GND (black). I was thinking I could use it just like a 'normal' FPV camera and get a video transmitter for it. However, it has four wires rather than three like most. I read that DM and DP are both data lines, but as a high/low pair or something. What do I do with these? Is the camera usable with something like this video transmitter?
AI: These four wires are most likely a USB connection, red and black are power, and the other two are USB data. That means you can't use it neither with today's analog gear, like the VTX that you linked to (as it requires an oldschool analog TV signal), nor any of the new and shiny digital FPV systems, like the DJI system (as they use their own proprietary protocols).
However, you can use it for flying if you really want or need it — you just need a computer on your drone that can process the video feed, such as a Raspberry Pi, and some kind of digital wireless connection, such as WiFi or even 4G to stream that feed to your phone or computer on the ground. That's how a lot of commercial drones work, and you can build something similar yourself.
Whether you really need or want it is another question. Using such a setup for the "sporty" kind of FPV flying, i.e. racing or freestyle, is probably a bad idea, as the latency will be very high, on the order of hundreds of milliseconds if not more. Such systems are more suited for long range and autonomous craft, which generally fly themselves (and sometimes even do some onboard analysis of the video feed), or just as a not necessarily practical but fun-to-make project to prove to yourself how cool of a hacker you are (or as part of such a project, like a facial-recognition patrol drone or whatever). |
H: Restore over-discharged LiPo batteries ~2.0V/cell
I have a LiPo battery which is ~2.0V/cell, how can I restore it safely? By "safely" I mean how can this be controlled as I know LiPo batteries can be dangerous.
AI: Firstly, I'll add the usual statement that mistreated LiPos can be dangerous and you are almost always better getting new ones.
In some cases, a 'smart' battery charger will refuse to charge cells which have dropped between a certain (manufacturer-determined) voltage. In this case, you can connect the battery to a current limited voltage source and charge the batteries manually with a very low trickle current - 0.1 to 0.2C is more than enough. It will take a while, but speed is not the aim here. You might be able to do this with the charger, by telling it the battery is a different type, or by using a separate supply. In either case, you must ensure the max voltage is within the normal range for the LiPo (4.2V/cell.)
You should only attempt recovery of a suspect battery in a safe, supervised environment - charging is the most likely time for such a battery to fail, and particularly so if it has been over-discharged. |
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