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In 1880, Pierre and Jacques Curie determined that a small amount of voltage could be produced by applying large amounts of pressure to certain crystals of elements. This phenomenon is called the piezoelectric effect. When pressure is applied to the crystal, it will deform and produce a small voltage. The amount of voltage is proportional to the amount of deformation. The best crystals that are used for this type of sensor come from ammonium dihydrogen phosphate and sintered ceramics. The amount of voltage that is produced is very small and the internal impedance of the crystal is very large, which makes the use of op amps a necessity to produce a usable signal.
When the piezoelectric effect is used in a pressure sensor, the sensor uses a diaphragm that deflects slightly when pressure is applied. A rod transfers this small amount of movement directly to the piezoelectric crystal. The pressure on the crystal causes a small voltage to be produced that is proportional to the pressure. The voltage is amplified to traditional voltage signal values (0-10 volts). The figure below shows an example of a piezoelectric pressure sensor.
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