Piezoelectric elements can be used to make sound or detect sound. Steve Hoefer has made a Knock Lock, which is a device that can listen to knocks on the door. If the sequence of knocks is the same as the recorded knock sequence it unlocks the deadbolt on the door. Steve was the guy who also made the Dice Reading Machine.
"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."
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