How the Motion Detector Works
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Beam Detectors
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Active motion detectors send out signals to detect motion. One of the simplest types of active detectors is a beam motion detector. A beam of infrared light is directed toward a light detector. When someone or something breaks the beam, the detector registers it and sets off an alarm. These motion detectors are ubiquitous in heist movies, where the thief often has to dodge and duck through a grid of lasers to steal a precious jewel or artifact. They also have a much more prosaic use: beam detectors are used in stores to set off a chime whenever a customer enters or leaves the building.
Proximity Detectors
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An active proximity sensor sends out a pulsed signal such as a high-frequency sound above the human range of hearing or a radar signal. This signal then bounces off a nearby object and back to the sensor, which records how long the signal takes to make the trip. If someone steps between the signal and its destination, the signal will bounce off that person. This will shorten the trip, sending the signal back to the detector more quickly. The detector will register this shorter trip and activate. These sorts of sensors are used to operate automatic doors. A signal is broadcast from the door to the sidewalk in front. When someone steps in front of the signal, it bounces off him back to the sensor, telling the door to open.
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Passive Detectors
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Outdoor safety lights and some indoor security systems are powered by passive infrared detectors. These detectors do not send out a signal. Instead, they wait for nearby people to send out "signals" of their own. Pretty much every object emits infrared light, which is invisible to the human eye. The frequency of infrared emitted depends on the temperature of the object. The human body emits infrared in a very narrow frequency range. Passive motion detectors have special light sensors designed to pick up light in that frequency range. When a person walks near the detector, it picks up her light and triggers a floodlight, alarm or anything else connected to the system.
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