If you take a look inside you’ll find an Arduino, a piezo sensor, a solenoid, and a nine-volt battery. The piezo sensor detects your knocking as an input.

If that sounds like a dream come true, then you might need one of these Secret Knock Detecting Door Locks. This innovative device was made using an Arduino, a motor, and a piezo sensor.

2. Piezoelectric Sensor: This sensor will act as your sound detector. When it picks up a sound, it sends a signal to the Arduino, which then triggers the camera and flash. 3.