I'm trying to use two servos simultaneously and I've connected both servos to my battery pack which gives 6V (the servos require 6v). If I ground the servo only to the battery and I only use one of them it works fine. If I want to use both, the servos just stop and one of them sometimes moves a little bit. I noticed that if I wire it also to the ground of the RPi (with a 1k Ohm resistor between) it works fine. I can use both of them simultaneously. So why do I need two groundings to work fine? Can't the single grounding handle both of the PWM outputs from the Pi or something in that direction?

1 Answer 1


You need to create a circuit for electricity to flow from the Pi to the servos and then back to the PI.

You control servos by sending 1-2 ms pulses along the control wire at 50-60 Hz.

If you don't have a ground wire between the servos and the Pi there is no circuit for the pulses.

  • So it's not possible to use another ground like the grounding of the battery pack? Why did it work with a single servo? Feb 2, 2016 at 21:43
  • The battery pack -ve is connected to the servo ground. They are equivalent. Connect either the battery pack -ve or the servo ground to a Pi ground. The answer I gave was overly simplistic. The ground is a common reference point for the the voltages. It might work without a common ground but it will not be reliable.
    – joan
    Feb 2, 2016 at 21:49
  • Can you explain to me why it is that way or link me to a source? Feb 3, 2016 at 12:46
  • The best I can do is point to a post electronics.stackexchange.com/questions/90580/…
    – joan
    Feb 3, 2016 at 13:21

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