unlocking a door with a motor

Im in the process of planning a new function to my homa automation. I want to unlock/lock my frontdoor. My first thought is to use a DC motor to turn the key, and a concern that has come up is that I wont know how many rotations I can go before the key is at its "end". Depending on how it was left the last time it could be 1,2 or three rotations. Anyone got any suggestions on how I can detect this, or any other suggestions on how I can use an Arduino to unlock my door?

You could use current feedback to determine motor torque and stop on, say, 80% of stall current + a short timeout. More detail?

This sounds like what I was thinking about. More details would be much appreciated

With small motors you can limit the max current by reducing the PWM drive - the resistance and other losses in the motor will then effectively limit the current, preventing risk of key snapping.

The key is on the exterior side of the door, correct? - Scotty

Sorry, I was asking for more detail from you, like how will you mount a small gearmotor and connect it to the lock mechanism, etc., the idea of monitoring current was to detect the physical ends of travel and limiting torque below damage level.

Illustration on what I was thinking

snorriengen: Im in the process of planning a new function to my homa automation. I want to unlock/lock my frontdoor. My first thought is to use a DC motor to turn the key, and a concern that has come up is that I wont know how many rotations I can go before the key is at its "end". Depending on how it was left the last time it could be 1,2 or three rotations. Anyone got any suggestions on how I can detect this, or any other suggestions on how I can use an Arduino to unlock my door?

I think this is like the difference between old-school analog, and new-school digital. New school digital would have the mechanisms (mechanical/electronic) built into the door itself.....so an electronic signal or code just unlocks the door and opens it.

Your method is more like a robotic method, which requires something to be mounted - possibly in some unwieldy way to the door knob......or the key.... in order to turn it. But I guess if a robot could be programmed to move up to the door, and detect the key, and is able to automatically grasp the key, and reliably open the door....that would be a pretty interesting project.

For detecting if the key is turned to some extreme, I guess a current or force sensor might be able to detect this condition. Like...if the motor turns and the current monitor senses an unusually high reading, then that could be a signal for backing off on the force.

Hehe. Well, if I were to change the lock I could just go for an out of the box solution wich I dont wont.

My "end-plan" is to make a box for this setup and place it over the key and using fingerprintscanner on the outside. Not having it take it self of and on.

snorriengen: Hehe. Well, if I were to change the lock I could just go for an out of the box solution wich I dont wont.

You could, but that doesn't mean you have to. You've already made clear that you don't want to use an out of the box solution. There's nothing stopping you from modifying the inside of the lock at that point.

It'd be much simpler to skip the whole rotation business and build a deadbolt lock from the ground up with a solenoid.

The goal is simply to move a chunk of metal along a line. This is simple.

They added on the rotating key part as a human interface, and you no longer want to human interface.

Your solution of using a motor to turn a key to move a bolt is like building a robot to push the actual buttons on a tv remote (instead of just giving the robot an IR emitter).

As an alternative to turning a key, you can buy an "electric strike lock" or an "electric lock".

There has already been a project for unlocking a door:

http://www.instructables.com/id/Arduino-RFID-Door-Lock/

scottyjr: The key is on the exterior side of the door, correct? - Scotty

Good point!

scottyjr:
The key is on the exterior side of the door, correct? - Scotty

Is that like the side that is like outside the home/building? Like… the key is exposed to the public world?