How to check 2, 3, 4 wire sensor : Electrical training
The sensors make it possible, for example, to check the position or the presence of a piece.
The inductive sensor detects ferrous objects while the capacitive sensor detects any material.
These detectors are active sensors. That is, they must be powered to working. These sensors must be tested with the power on.
The sensors can have 2, 3 or 4 wires. They are PNP or NPN type transistorized. "