This safe has a programmable door lock that responds to a secret knocking pattern, such as shave and a haircut. New knocking patterns can also be recorded. The door will open as long as the rhythm is correct, regardless of the tempo.
I built this as my final project for 6.115 (Microcomputer Project Laboratory), taken in Spring 2016 at MIT. It uses an Atmel 2051 microcontroller to filter a signal from a piezoelectric vibration sensor. The data is analyzed on a Cypress PSoC 5 board. The door then opens by retracting a solenoid lock.