I'd like to convert my stupid door lock into a smart one.

I already have a gear for the lock knob. Now I'd like to design a gear decoupling system that will be connected between the knob and the motor so that the door can be opened with a key when the motor is not running. I thought about using the electromagnetic clutch, but they seem to be not very popular because I could find only used ones from printers at an affordable price. I don't even know how to power them. I want to use something more accessible than this particular electro-clutch.

My second idea is to use a solenoid or servo to connect the gear racks together. It would be also cool if it was possible to design this using only mechanical parts. Does anyone know of any examples of projects with such a solution that I could take a look at? What are the ways to solve this problem?

  • 1
    $\begingroup$ How would you solve this problem? has many answers ... that makes it an opinion based question that is not allowed here ... please ask a focused question that would have minimal number of correct answers, preferably one answer $\endgroup$
    – jsotola
    Commented Jan 29, 2023 at 23:01
  • $\begingroup$ I remember seeing a question here, about a year ago, that dealt with a knob ... I think that it was a motorized telescope aiming knob that contained some type of planetary gear arrangement ... I'll try to find it $\endgroup$
    – jsotola
    Commented Jan 29, 2023 at 23:05
  • $\begingroup$ second picture here ... engineering.stackexchange.com/questions/2934/… ... remove the spring ... add a light spring in opposite direction ... friction between motor shaft and the arm causes idler gear to engage when motor spins $\endgroup$
    – jsotola
    Commented Jan 29, 2023 at 23:12
  • $\begingroup$ @jsotola but that only works in one direction, right? $\endgroup$
    – Defozo
    Commented Jan 30, 2023 at 2:15
  • $\begingroup$ how about a centrifugal clutch? $\endgroup$
    – joojaa
    Commented Jan 30, 2023 at 5:06

1 Answer 1


If you design your motor and gears with low enough friction, you can simply allow them to be backdriven by the mechanism when the key is turned, and there is no problem.


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