Knock to unlock

Via hackaday I found an automatic door opener (or unlocker) which responds to a specific knock pattern -- a knock code, I guess.

I like this because it's a disappearing computer -- an excellent example of real-world ubiquitous computing, as coined by Mark Weiser. Obviously the hack doesn't look great or elegant, but I can certainly envision a version which has entirely disappeared inside the door. You knock correctly and the door opens and you never think about how it works. (Well, someone never thinks about it. I'm sure you and I will.)

update: Apparently this tech has already been commercialized by Knock N'Lock, which touts it as a new level in security. (It's "a lock that cannot be picked or tampered with.") I'm now starting to wonder how hard it would be to build a little knocking device to bruteforce a door...