Inductive proximity switches are used for instance for measuring motor speeds or determining the position of metal objects. They do not suffer from mechanical wear or contacts that spark. The latter is particularly important in spaces where explosive materials are stored.
Virtually all commercial proximity switches are constructed as shown in fig 1. An inductor in the resonant circuit of an oscillator serves as the sensor. If a conducting object enters the magnetic field of the coil eddy currents are set up in the inductor. This damps the resonant circuit so that voltage across it drops. This voltage drops monitored with a Schmitt trigger. When the objects get very close to the inductor and voltage across the circuits drops sufficiently the Schmitt trigger changes state. The trigger is followed by an output stage.
The sensor IC1 used in present circuit translate the approach of an object into falling current through the sensor. In the absence of an object, the current is about 4mA when an object is at the distance of 4mm from the sensor the current is only 1mA.
The sensor current is converted into a voltage by R1. This voltage is applied to the non-inverting input of Schmitt trigger C2. Actually, since the hysteresis is small. functions more as a comparator that likens the voltage across R1 to that across R3. When an object is within the proximity limit of 5mm of the sensor the potential across R3 is larger than that across R1 whereupon the output of IC2 becomes logic high.
The potential across R3 is dependent on the supply voltage of course but over the range of supply voltage stated in the diagram, it is always greater than the smallest drop across R1 and always smaller than the largest drop across R1.This ensures the correct switching of IC2 in all situations.
The only quantity affected by the supply voltage is the output potential of the circuit.
The is a Type IFR10-82-01 from Baumer Electric. It has a diameter of 10mm and is 5mm long.