What is the function of piezoelectric crystal?

Answer 1

Piezoelectric crystals have unique properties. If you strike them (not too hard), they produce a high voltage pulse. If you apply electricity to them, they swell. If you just tag them with a pulse of electricity, they ring at their modal frequency. These properties allow them to be used in many applications. The following are just a couple examples of their use.

Disposable lighters that don't have flint use a piezoelectric crystal to generate an arc that ignites the gas.

Accelerometers use piezoelectric crystals to generate signals proportional to how fast something is accelerating.

In electronics, piezoelectric crystals are used to generate master timing signals.