I was playing with getting my servos to be on dedicated power, separate form the Arduino entirely, but obviously controlled by the Arduino. I originally had the ground of the Arduino joined with the - of the battery, but to my surprise my Arduino continued to control the servos when I remove the ground wire entirely from the Arduino.

I was under the impression that all electricity must form a complete circuit, and be a closed loop. But my Arduino can obviously send a signal to a component that has no return path to itself.

I feel like I'm missing something fundamental about a electricity. So my question is:

Why does this work?

arduino setup

1 Answer 1


The return signal is probably the other wire's servo. This only works when the control pulses to both servos are not in sync. Pull one signal wire off the breadboard and both servos will stop working.

You need to interconnect grounds or you will get strange effects sooner or later.

  • Are the I/O-pins connected to GND when in OUTPUT mode and not outputting? This would make sense, and would mean this answer is correct. Commented Apr 13, 2014 at 12:50
  • @FriendofKim In any output mode through protection diodes in the microcontroller an in OUTPUT node through the driver transistor.
    – jippie
    Commented Apr 13, 2014 at 14:04
  • Yep, it doesn't work with just one servo, which I didn't think to try. Thanks!
    – Alex Wayne
    Commented Apr 14, 2014 at 4:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.