For my application, I need to be able to control a servo with a simple on/off switch. When it's off (pulled LOW), I need to servo to go at -90 degrees and stay there - possibly for a prolonged time. When it's on (pulled HIGH), I need the servo to go to +90 degree and stay there - possibly for a prolonged time.

I'd like to minimize the control logic footprint and complexity, and also to minimize standby power consumption in servo jitter in both positions. So ideally I'd want to get by without Arduino at all - with some kind of a simple analog / IC comparator logic based on servo's internal potentiometer, and direct control of motor wires.

Rotation precision is not important, so +90 can be "anything above +85 degrees", and -90 can be "anything below -85 degrees".

I won't be able to fit external end-switches to the mechanical application and want to rely on servo's internals only. I have a bunch of no-name SG90 lying around (which are a perfect fit size-wise and torque-wise) - but I would not be against buying a different kind of a "servo-like" actuator that would facilitate a simpler conversion.

Can someone suggest where to look for the solution schematics - and whether it might exist as a pre-made IC?

I found a Servo Trigger board: https://www.sparkfun.com/products/13118?_ga=2.160099069.1169016740.1587923297-1103320110.1587923297 which at a first glance does what I need ad is easy to use. But it is still an over-complicated (internally) solution with a constantly running microcontroller to produce PWM pulses that are to be decoded on the servo side. I guess a much simpler (and less idle power draw) solution would be possible based on a direct hook-up to servo's potentiometer and analog comparator logic, but I can't find anything.

Will appreciate any advice.


  • $\begingroup$ start by determining what is inside the no-name SG90 $\endgroup$
    – jsotola
    Apr 26, 2020 at 23:11
  • $\begingroup$ Great point. There is a 3-pin potentiometer that i can easily access to (i’d guess i’s Vin, Gnd and Vout as it is with most other potentiometers). The it’s 3 wires going to the motor, which functions i’d need to determine (or find out the markings on the motor) - TBD. And a control board with one IC in between. $\endgroup$ Apr 27, 2020 at 2:28
  • $\begingroup$ an external controller will allow you to replace a failed servo without first making modifications to the replacement servo $\endgroup$
    – jsotola
    Apr 27, 2020 at 17:14
  • $\begingroup$ Welcome to Robotics Dmitry Alergant, but I'm afraid that Unbounded Design Questions are off-topic because there are many ways to solve any given design problem. We prefer practical, answerable questions based on actual problems that you face, so questions which ask for a list of approaches or a subjective recommendation on a method (for how to build something, how to accomplish something, what something is capable of, etc.) are off-topic. Please take a look at How to Ask & tour for more information on how stack exchange works. $\endgroup$
    – Ben
    Apr 28, 2020 at 0:30

1 Answer 1


Make a 555 servo exerciser, and replace the position pot with your switch (or use a pair of position pots and switch between them).

Here's your starting point: http://www.seattlerobotics.org/encoder/200210/servoex/ServoExcerciser.htm

  • $\begingroup$ And make sure the servo will do +/-90 degrees -- nominally they're only good for +/-45, but most will do the full 180 degree swing. $\endgroup$
    – TimWescott
    Apr 27, 2020 at 15:29

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