Thursday, March 12, 2015

Pocket oscilloscope for measuring rc servo pwm

It's been some time since I looked at the robot. A few things have changed. I'll comment on them later. But first, one of the troubles I had was determining if the servo signals my Pulse Width Modulation (PWM) board was putting out were correct.

I had been looking for servo testers etc in the RC hobby world and couldn't find any. It looked like getting an expensive oscilloscope was going to be the only way to solve this problem. I checked usual places for used equipment. The cost was crazy. This is supposed to be a cheap fun project.

I stumbled upon this little device. It's a pocket Oscilloscope selling on Amazon for around $50.

So far so good. In the picture you can see the signal being sent of approx 1.5ms knowing pwm servo signals vary from 1 to 2ms this looks reasonable for a center position. You can change the timebase of course and get a better signal view. I'm impressed. Solved the problem. At least now I can see if my code is sending out what I expect.


My PWM controller is the Adafruit 16-Channel 12-bit PWM/Servo Driver - I2C interface - PCA9685. I had hoped that I could use the scope for also monitoring the I2C interface. Now that java is supported better on the Raspberry pi (I'll talk about that in another post) I thought I would see how effective it would be to write the code in Java since that's my daily language. That means creating a java PWM library for the Adafruit controller. It would be good to be able to test that out with the pocket scope. However since it can only handle one trace at a time it's not possible to monitor both I2C signals.


General Servo pulse ranges.


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