Holy shit, it's a PVC pipe bomb! No, not really. It's the Knock Lock, a homebrew door lock that will only release the deadbolt if you perform the secret knock. Cooooool -- I want one for my clubhouse!
A microphone (okay, really a speaker) presses against the door and listens for knocks. If it hears the right number of knocks in the right cadence it triggers the motor to turn the deadbolt and unlock the door. If the sequence isn't recognized, the system resets and listens for knocks again.
There's a very worthwhile video after the jump of the lock in action. The only problem is every time you perform a knock your neighbors learn how to gain access to your apartment. Still, neat idea. But I'll just stick with my tried and true knock: KICK IN THE DOOR, WAVIN' THE FOUR-FOUR, ALL YOU HEARD WAS GW DON'T HIT ME NO MORE!! I'm serious, don't make me pistol-whip you.
Hit the jump for another shot and the video.
Secret Knock Detecting Door Lock [grathio]
Thanks to EggL, whose door requires a retinal scan plus blood test.