A chunk of piezoelectric material can be used in many ways:
- To convert mechanical energy into useful amounts of electrical energy ("energy harvesting")
- To convert mechanical vibrations into detectable amounts of electrical signals ("sensor")
- To convert electrical energy into mechanical vibrations (perhaps the most common application; piezo buzzers and piezo speakers)
- To convert electrical energy to mechanical vibrations for a short time, and then immediately afterward convert mechanical vibrations still in the crystal into electrical signals directly, with the mechanical energy never leaving the crystal (quartz oscillator crystals and high-voltage-generating piezoelectric transformers)
- To convert electrical energy to mechanical vibrations for a short time, letting the vibrations exit the crystal, and sometime later listen for echos of that vibration to return to the crystal, which converts them back to electrical signals (as in pre-birth ultrasound testing and ultrasonic range finding (SONAR) and fish-finders).
- See also energy harvesting power sources
- One of many kinds of sensors.
driving a piezo disk
A piezoelectric disk acts much like a capacitor. (As opposed to, say, a coil speaker, which (electrically) acts like an inductor).
Say you drive the gate/base of a transistor from an oscillator, and hook the other pins of the oscillator and the 2 drive pins of the piezoelectric disk to each other and power.
When the transistor turns on, it charges that capacitor up. But when the transistor turns off, you need something else in that circuit to discharge that capacitor.
Some people put a resistor from the collector of the transistor to the +V power supply. 
Other people get a much louder sound because they put a small inductor from the collector of the transistor to the +V power supply. ; (United States Patent 6,407,507).
A few people get an even louder sound using a transformer from the collector of the transistor to the +V power supply. 
Some low-cost atomic microscope plans call for a piezoelectric disk.
- Main page: oscillator