What to do if you are renting a room in students flat and want to make use of an unused, empty room? You just walk in and do your thing. What if the landlord, for some stupid reason he got from his higer-ups, has to lock all unused rooms? You go ahead and pick the lock. Not really 100% legal, but closing an empty room for no apparent reason isn't really ethical too, in my opinion. But what if the landlord comes to check if the room still is locked? Hmm...
A friend of mine happened to live in a students' flat where such a thing happened: the landlord basically made them cram a bunch of stuff, which usually was in the main hallway, into a room that was too small for it, while in the same flat three rooms were completely empty, but locked. Luckily, the guy could lockpick his way into a room, but he asked me for a way to be able to re-lock it without the key and without anything suspicious being visible from the outside.
After some brainstorming, we came up with a James Bond-worthy concept: knock on the door in a certain fashion, and the door'll open automatically. The idea was perfect: no need to drill holes, sneaky enough so the landlord wouldn't accidentally trigger it, easy enough to remember.
The implementation was a bit more difficult: We needed something mechanical to open the lock from the inside and something to detect the knocks themselves. The first of the two wasn't too hard: the lock has a 'knob' on the inside, so you can lock and unlock the room from the inside without having the key. The second probably could be solved with a microphone, a small amplifier and a microcontroller to detect the knocks and see if they're in the right order.
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