Here’s a fantastic project using an arduino, a motor, and a piezo sensor which allows you to both record and detect knocking patterns to unlock a door. The video below does a fantastic job of demonstrating how it works: It’s completely mobile and can be suction cupped to any dead bolt. Press a button to record a new sequence. Upon detecting the correct sequence of knocks, the motor turns the lock open. Brilliant!
- View the Full Tutorial
More Details: Secret Knock Detecting Door Lock