In general, inductive proximity switches consist of four basic elements: a coil, an oscillator, a threshold switch and an output stage with short-circuit protection.
The oscillator generates a high frequency, electromagnetic alternating field which is emitted from the active face of the coil.
Eddy currents are induced in a metal object that enters this field. These eddy currents draw energy from both the electromagnetic field and from the oscillator which is consequently attenuated.
The more energy taken the closer the metal object moves towards the active face. The threshold switch switches on the output stage at a defined attenuation value.
In proximity switches with a DC voltage supply, this switch is designed as an NPN transistor which switches the connected load to the negative pole or as a PNP transistor which switches the load to the positive pole. The output stage is a thyristor or a triac in AC voltage switches.