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


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. Apr 13 '14 at 12:50
  • @FriendofKim In any output mode through protection diodes in the microcontroller an in OUTPUT node through the driver transistor.
    – jippie
    Apr 13 '14 at 14:04
  • Yep, it doesn't work with just one servo, which I didn't think to try. Thanks!
    – Alex Wayne
    Apr 14 '14 at 4:48

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