Knock Lock – Unlock a Door with a Secret Knock
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. Via: Zedomax "While working on another project I