I would like to automate a door lock. For reasons I won't get into, I'm not allowed to modify the lock itself, I can only attach things to the "safe side" of the door. On this safe side, instead of a keyhole, there's a knob. I've already attached a continuous servo to this knob (the lock needs multiple turns between its locked and unlocked position). The problem now is that when opening the door manually with a key from the other side instead of via the servo, the microcontroller loses track of what position the servo is in.

I would like to solve this with limit switches in the locked and unlocked position: that way, the microcontroller can instruct the servo to keep turning until the limit switch is depressed. If the lock needed less than a single turn between locking and unlocking, this would be easy to do, but because it need multiple turns, I can't really think of a way to do this.

I am looking for a way that allows two limit switches. I considered using a multiturn potentiometer, but decided against it because I would like to limit additional torque.

An ideal solution would be low-torque, cheap and ideally 3D-printable.

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    $\begingroup$ Use a multi-turn potentiometer as a cheap absolute encoder? $\endgroup$ Jan 24 at 14:14
  • $\begingroup$ Use a screw thread to covert the rotation to linear movement, and use a linear encoder? $\endgroup$ Jan 24 at 14:15

There are many options:

  • a gearbox
  • multiturn potensionmeters
  • convert rotational to linear motion through a screw
  • digital encoders (absolute or incremental)

If you need less that 5 or 6 turns you can use a gear box with a single turn stop. The gear box will reduce 5 turns to 1.

Regarding the use of multiturn potensiometer, IMHO is a viable option. The added torque is not significant and the cost is quite low. The only problem, is that they are quite flimsy - I've used one and after a test it actually broke off.

enter image description here

Another solution is using a screw which translates a solid block which activates the switches.

enter image description here

Finally, since you are already using microcontrollers, you could use software switches. You can either use:

  • absolute rotational encoders (these are expensive), or
  • incremental encoders with z index channel: which are much more cost effective.

enter image description here


You could try this mechanism (please pardon the mspaint drawing):

Notice how the inner shaft has to rotate more than 360 degrees to hit both limit switches. You can use multiple stages to get more rotations.

mechanism drawing

  • $\begingroup$ Neat! Does this mechanism have a name? Also, won't the outer part depress the switch for a while until the inner part hits the other side of the inner side of the outer part? $\endgroup$
    – redfast00
    Jan 24 at 12:13
  • $\begingroup$ That's a bit like two stages of a multi-stage rotary dial combination lock although I think they are usually arranged axially rather than radially. The long on-time of the switch during reversal won't matter as it is only limiting travel in the "wrong" direction - unless you were to start to unlock and then relock in which case it wouldn't lock fully. $\endgroup$
    – Transistor
    Jan 24 at 14:11

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