There is a simple lock mechanism connected to servo which holds door closed. When you set correct passcode with dimmer, the door will be opened and lights inside the safe will be turned on. After you close the door, lock mechanism secure safe again.

Watch this presentation video: http://youtu.be/yedk1cwqeQw

Project page on littleBits site: http://littlebits.cc/projects/perfect-safe

Technically there is Arduino littleBits module with custom code.

Passcode is read from dimmer module connected to analog input. User just rotate to correct position (number from scale 1..9) and wait for a while. There is internal buffer filled every 200 ms with number sampled from analog port. When sampled value is stable for 1.5 second, it is taken as one of the passcode number, stored in RAM and Arduino beeps through buzzer module (to give user feedback that number was taken).

After each given value passcode is compared to PASSWORD constant. If they are same, servo is set to HIGH state and lock mechanism open the door. In the same time, internal LED is turned on (because you want to see content of the safe).

There is also simple switch inside, so after you take content from safe and close the door, switch is turn on, Arduino turn off the ligths and secure lock with the help of servo.

Servo module sometimes does not play nice with Arduino and older batteries (see this thread). If you will have troubles with vibrations, change first batteries with fresh one. If your troubles remains, contact littleBits support please.