The ability of piezoelectric devices to transform mechanical energy into electrical energy makes them a natural choice for a sensor. A piezoelectric device installed in a building or bridge has no output as long as the pressures around it remain unchanged. When weight shifts, the piezoelectrical device emits an electrical signal that can trip an alarm. The device needs no batteries--it produces its own electricity. Piezoelectric sensors are cheap enough and rugged enough so that they can be placed one at the end of each I-beam in a building. At any time--from a day to tens of years later--you can tell from the sensors if any of the I-beams has had a load shift--a sure sign that some part of the building is failing.
Piezoelectric crystals have many useful properties. If you connect a piezoelectric crystal to a device (like a capacitor or a coil) that temporarily stores electrical energy, it is easy to create an oscillator--something that vibrates at a fixed frequency. Because the size of the crystal determines the frequency of the oscillator, piezoelectric crystals make an excellent basis for a timekeeper. Piezoelectric crystals made the cheap digital watch possible. The watches were cheap and reliable because they had no moving parts (unless you count the electrons). The regular vibrations of the piezoelectric oscillator are digitally processed to produce the digits on the display screen.
One use of piezoelectric material is currently a subject of medical research--synthetic skin. When a patient has had a large amount of skin damage, there is sometimes an area that is relatively devoid of sensation--the patient can't feel anything that happens at the site. This condition (dysautonomia) is dangerous. We have pain sensors to warn us of danger, and when the warning does not come, serious damage may be imminent. Restoring sensation to the skin is the best solution to this problem, but that would mean using a technology that turns mechanical energy into electrical energy. Piezoelectric film is a plastic film that has wires attached to it. Touching the film anyplace sends current to the wires depending on where the film is touched. The use of piezoelectric film as a way to restore feelings to damaged skin is still under investigation.