A proximity switch works by emitting an electromagnetic field and monitoring it, activating whenever a sensor detects a change in the field. The activation of the sensor then sends an output or an electrical signal to switch on a light, alarm or device.
Several types of proximity sensors exist, as different types are used to sense different kinds of objects. An inductive proximity sensor specializes in detecting metal objects like steel, brass and aluminum as they come into range. Capacitive sensors are more receptive to anything entering their fields, and can be used on things like plastics, liquids and even humans. There are also sensors for detecting magnets entering and leaving the field of the sensor.
Proximity switches have a wide variety of uses in industry and in everyday life. They are used in parking sensors for cars to alert drivers of low obstructions behind the bumper and in planes to detect the distance from the ground during landing. Inductive sensors are used for things like conveyor systems and in certain types of industrial piston engines. Small capacitive proximity sensors are used liberally in our everyday lives, such as within trackpads and touchscreens to eliminate accidental inputs. Smartphones also use proximity sensors to reduce radioactive emissions when the phone is close to people.