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.