Secret Knock Detecting Door Lock - Made by Steve Hoefer
http://grathio.com/2009/11/secret-knock-detecting-door-lock.html

Weil Türen durch geheime Klopfzeichen aufgehen sollten.

While working on another project I ran across the Arduino knock sensor tutorial. Sensing a single knock is a great little project for learning about microcontrollers, but what about sensing specific knocks? Seeeeecret knocks? And if we could detect a secret knock, shouldn't it unlock a door? If you can't tell by looking this was cobbled together from spare stuff around the lab, it's not much more than a piezo speaker, a tiny gear reduction motor, and an Arduino. And PVC pipe.

"Seeeeecret knocks? And if we could detect a secret knock, shouldn't it unlock a door?" (via @yetzt)

Very neat contraption that can unlock your door based on a programmed knock. Not very practical but still pretty awesome.