The piezoelectric effect works by generating a charge when the material is deformed. The effect also happens in the opposite direction where a charge can deform the material. This property allows for the use of piezoelectric disks in both microphones and speakers. However, this video presents a new use of the disk, a Nerf triggered Alarm Clock.

The Arduino based alarm clock senses the hit from a Nerf dart on its target and deactivates the alarm, testing your early morning Nerf skills.

Via Normal Universe