A proximity switch is a device which detects when an object is nearby. When an object gets within a certain distance, it will turn on a circuit to perform some basic function, such as opening a door or turning on a light or an alarm. Some proximity switches are designed to detect certain kinds of objects, while others will detect pretty much anything.
An active proximity switch detects objects by sending out a signal and monitoring reflections. For example, infrared detectors shoot out a beam of infrared in a certain direction and wait for a signal to bounce back. The closer an object is, the quicker the infrared signal will bounce back. If the reflection comes quickly enough, the proximity switch activates a circuit. For example, an automatic door shoots a beam in front of it. If someone steps in front of the door, he reflects the beam, causing it to open.
A passive proximity switch simply monitors an area, waiting for something to change in it. One example is the passive infrared switch which controls some outdoor floodlights. All objects emit infrared light, but the frequency of the light depends on the temperature of the object. Floodlights have an infrared detector cell tuned to the frequency the human body creates. When someone steps in front of the infrared sensor, it detects her body heat and turns on the lights.
