How Do Capacitive Proximity Detectors Work?
Capacitive proximity detectors rely on the fact that the capacitance is dependent on the dielectric of the capacitor. Dry air has a dielectric constant close to 1.006, which the human body has a dielectric constant of about 80. Thus, when a human enters the field of a capacitor, the capacitance will increase by up to 80 times the former value. The electrical flux between the sensor wire and ground thus increased dramatically.
Several different types of instruments and circuit utilized the capacitance phenomenon, i.e., LC oscillators, Electrometers, and Bridge circuit.