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.
Using Piezos as Sensors:
To use a piezo as a sensor, it is necessary to devise 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.
The electrical equivalent circuit for this sensor is:
The voltage source represents the voltage that develops due to excess surface charge on the crystal. The capacitor represents the metallic plates. The load resistance results from the act of measuring the voltage across the terminals.
Piezo sensors are only useful for measuring dynamic forces, as the capacitance blocks direct current.
A/D conversion is necessary, since the piezo circuit produces a voltage output.
Low-pass filtering and output buffering/amplification will most likely be necessary as well.