I have bought an electrical appliance which has buttons sticking out of it. I want to interface with these buttons from a microcontroller, but the buttons are baked on a flex PCB in the underlying circuitry. I can not get to their pins, partly because I'll void warranty, and therefore I decided that a mechanical solution would be best. I would've much preferred an optoisolator wired to the signal lines, but it showed that it can't be done.

I measured the force needed to press the button and calculated that it would be around 4 Newton. (Placed object on scale, zeroed and measured weight while pressing button)

Because the electrical appliance is quite small (approx. 60x60x60mm) and my power supply is limited to 5V 2A I started looking at linear solenoids within this range and found that they are unable to deliver the required force. I then thought of using a stepper motor with gearbox, but this feels a bit overkill.

What are some recommendations to interface with either the physical button or its underlying circuitry, keeping in mind the constraints?

  • \$\begingroup\$ "I can not get to their pins and therefore I decided that a mechanical solution would be best." I question your decision here. It should be possible to interface to the buttons, they might be on a flexible PCB but they also need to interface to something that is (much likely) soldered to a standard PCB. Can you post some pictures of your device? \$\endgroup\$ Commented Sep 4, 2017 at 10:08
  • \$\begingroup\$ I would love to but another reason for not wanting to open it, which I should make more clear in the question, is that I will void warranty \$\endgroup\$
    – Zimano
    Commented Sep 4, 2017 at 10:09
  • \$\begingroup\$ "which I should make more clear in the question" - You can still edit your question and fix that. \$\endgroup\$
    – Dampmaskin
    Commented Sep 4, 2017 at 10:13
  • 1
    \$\begingroup\$ @Dampmaskin I know but I was stuffing a sandwich in my face! Will correct it shortly. \$\endgroup\$
    – Zimano
    Commented Sep 4, 2017 at 10:14
  • \$\begingroup\$ I think that externally (mechanically) interfacing to those buttons will be unreliable as hell. In some cases you'll end up in missed hits, especially on the appliance side. \$\endgroup\$
    – next-hack
    Commented Sep 7, 2017 at 6:47

1 Answer 1


Assuming the button travel is not too much, you should be able to find a hobby servo that can generate that much force at the end of a short arm within your power requirements. This has the added advantage of being very cheap and easy.


If a hobby servo will not work for this because of geometry constraints, then a photo of the buttons configuration might get better answers.

  • \$\begingroup\$ Tried it out using one I found at home on a scale and 4N was reached easily. Without any reduction/leverage the circular motion is something to consider in my final design \$\endgroup\$
    – Zimano
    Commented Sep 11, 2017 at 7:18

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