Temperature

A pressure applied to a polarized crystal produces a mechanical deformation which in turn results in an electrical charge. Piezoelectric microphones turn an acoustic pressure into a voltage.

Alternately, an applied charge will produce a mechanical deformation, which can produce an acoustic pressure (piezoelectric speakers).

The alignment of the electric dipoles in the piezo crystal produces an excess surface charge. This surface attracts free charges from the surrounding atmosphere to become electrically neutral.

A deformation disrupts the electrical dipole orientations, producing a temporary excess surface charge and a resulting voltage.

To use a piezo as a sensor, it is necessary to device a means of measuring its surface charge. One such method consists of sandwiching a piezo between two metal plates to make a capacitor. An applied force will produce a voltage V = Qf / C, where Qf is the charge resulting from the force and C is the capacitance of the device.

Sensistive to electrostatic pickup -> Shield it or use a instrument/differential amplifier to clean signal.

Driving Piezos with Transformer:

Driving Piezos by stacking them: