Proximity sensors are sensors that detect when a target object such as a metal has approached (or receded), without making direct contact with the object. Principle of high-frequency oscillation type: The detection coil located at the front end of the sensor produces a high-frequency magnetic field. When an object (metallic) approaches this magnetic field, induced currents flow in the metal, causing thermal loss and resulting in the reduction or stopping of oscillations. This change in state is detected by an oscillation state sensing circuit which then operates the output circuit.