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I'm trying to rig up my door to be unlockable using RFID and an Arduino. The problem is the actual mechanics of unlocking the door. There's a few limitations:

  1. I can't actually unlock the door. It's permanently locked and can only be opened from the outside with the key.
  2. It's a handle, not a knob
  3. It's an incredibly stiff handle, so any mechanism that would actually turn the handle would need a significant amount of force
  4. I'm renting the place, so I can't make any major or permanent modifications to the door or the door jamb or anything.

I tried using a motor and threaded rod to make a linear actuator, but that's slow and loud. I'm trying to consider whether it would be practical to put a tiny solenoid in the hole where the door's bolt goes when it's closed. (The hole in the strike plate in the door jamb) The hole is much larger than it needs to be (1" deep for a 1/2" bolt), so theoretically, if I had a solenoid that's 1/2" long when unpowered, it would fit in there, and could directly push the bolt so the door could be opened without turning the knob.

Unfortunately, I don't have any experience with this sort of thing, so I have no idea if this is even remotely practical. What would you recommend? Is a tiny solenoid a reasonable solution, or am I better off finding a way to physically turn the door knob?

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  • \$\begingroup\$ I very much doubt you can find a solenoid that can double its length in that form factor. \$\endgroup\$ – pjc50 Dec 16 '14 at 9:36
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When I was studying at the university, we had a similar problem at the electronics club. The management didn't want to give as many keys as we would have needed, and the keys were not copyable (at least legally). We solved the issue by making a card reader and then we used an air compressor and an electrically controlled valve to blast a piston to the end of the door handle. Worked like a charm.

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First, several comments:

  • Solenoids generally pull, not push. It's hard to imagine that one could fit in the space you describe and still develop enough force to move the bolt.

  • Requirement #4 is a killer. As you probably know, you'd normally just install an electric door strike in the jamb, but that's a permanent modification.

Given your constraints, I would consider getting a replacement lockset for the door and modifying that with a solenoid or motor that can pull directly on the bolt from the inside. Save the original lockset so that you can replace it when you leave.

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