Read RC PWM signal with Arduino

Arduinos are cheap and simple development board. You can do a lot with even the simplest of them. For example build you own quadcopter and flight controller (after all MultiWii = Arduino + MPU6050). Of course, this is not as simple as one might imagine and there are few (actually a lot) obstacles that needs to be overcomed. One of them, and very basic, is how to read RC PWM signal provided by radio receiver.

Signal to decode

RC PWM signal passed from radio receiver to servos, ESC, flight controllers is encoded with a length of pulse. Pulse length of 1000us (micro seconds) is minimum stick position and pulse of 2000us length is maximum stick position. Pulses repeat every 20ms for standard 50Hz refresh rate. Like this:

RC PWM Signal

So far, nothing fancy. Continue reading “Read RC PWM signal with Arduino” »

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Note on Arduino Uno servo jitter

Yesterday I discovered very nasty feature of Arduino Uno (and all other AVR ATMega328 boards) when using servos. Although official Servo library states that it can support up to 12 servos on Arduino Uno (more on advanced boards as Mega), it does not say much about quality of PWM signal.

Since all connected servos (in case of Arduino Uno/ATMega328) are driven using the same timer (timer1), the more servos are connected, the more jitter is introduced to PWM signal. Control “window” of each servo starts to overlap. This results in a situation when real pulse width jumps up and down, sometimes even outside allowed values.

My experiments says that Servo library can support up to 3 servos per 16 bit timer with acceptable jitter level to use as RC control signal. Specially when PWM signal is fed to flight controller. With 3 channels signal quality was acceptable after enabling input filtering on Cleanflight. 2 PWM/Servo channels did not required input filtering.

4 or more PWM channels can be used when real servos, not flight controller inputs, are used. Servo inertia “solves” issue of signal jitter.

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Using transistors as switches

I think all popular computerized DIY devices like Arduino, Raspberry Pi or any other microprocessor/microcontroller based boards has one common drawback: low output current. Few miliamps per pin. While this is enough to light a single LED or provide input to other electronics device, it is far from enough to run a motor or power a LED strip. It’s all about current.

Good thing this problem can be solved with two additional devices: resistor and bipolar transistor. Together they can act as a switch. Idea is simple: low current (and voltage if you wish) applied to transistor base causes bigger current (and voltage) to be passed between collector and emmiter. We have two choices: NPN or PNP bipolar transistor. Switch that uses NPN transistor is open/enabled when positive voltage is applied to base. In other words, base is connected to plus.

NPN transistor as switch Continue reading “Using transistors as switches” »

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How to flash MW OSD to Micro MinimOSD

While playing with newly arrived Micro MinimOSD and attempting to flash it with latest MW OSD software, I’ve discovered that offical tutorial, as well as many other are missing one tiny detail that makes whole process much harder than expected. In theory, MinimOSD just Arduino Pro Mini with additional hardware. So, in theory, you plug in your FTDI USB adapter and upload a sketch. But if you never worked with Arduino Pro Mini (like me, it was always Uno, Pro Micro, or barebone Atmega328) you might not know one detail. Yesterday it took me more than 30 minutes to figure out firmware upload is not working, while it should. So, another tutorial will be written.

Micro MinimOSD with soldered pins

Micro MinimOSD pinout

 

To flash MW OSD to Micro MinimOSD we will need:

Continue reading “How to flash MW OSD to Micro MinimOSD” »

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Simple servo tester with Arduino

Servo tester is a useful little device that is needed specially when you do not have one. I’ve been placed in a situation like that few days ago what I was trying to connect ailerons with servos on my Depron plane and setup correct control throw. Luckily, I own a few Arduinos,  so 15 minutes later I owned a divice like that:

Servo tester with Arduino

It is an extremely simple servo tester based on Arduino Pro Micoro that can send 3 different PWM pulse lengths: 1000us, 1500us and 2000us. That gives min, neutral and max stick position on RC radios. Perfect to setup control throws and neutral positions.

Required parts:

  • Arduino. Any Arduino will do. I used Arduino Pro Micro, but any can be used. Even barebone ATmega328 running internal oscillator on 8MHz,
  • Breadboard,
  • Tact switch,
  • Some golpins to make servo connector,
  • Cables.

Electrical diagram goes as follows:

Arduino servo tester diagram

Program is also not complicated:

And at the end, it works like that:

Source code is available on GitHub.

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