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There are five basic types of motion detectors, and the active infrared (IR) and passive infrared (PIR) detectors are the most common types used. Other versions or types are the continuous wave (CW) radar, ultrasonic motion and vibration motion detectors. The main difference between active and passive types of motion detectors is that active detectors send out signals and search for motion, while passive detectors do not emit any signal. A passive detector reacts to a signal that is sent to the detector, such as noticeable body heat or sound waves. An active detector, however, sends out a pulse of ultrasonic sound waves and measures the reflection of the waves to detect motion.
Many motion detectors are used as switches on lights and alarm systems. By utilizing motion detectors, motion-sensing lights can be used to illuminate an area when a suspected intruder is present. While referred to as a motion light, it is, in reality, a light triggered by a motion detector. This type of system uses passive detector technology to identify motion. Used on lower-cost illumination and alarm switches, the passive system relies primarily on detected body heat to trigger a switch on the light or alarm, indicating someone or something has been detected in the vicinity of the sensor.
Higher-end motion detectors rely on active detection methods, such as ultrasonic sound-waves, to detect the presence of motion. This technology sends out sound waves and measures the time it takes for the waves to reach a known distance and return. When an intruder enters the midst of the sound waves, the time it takes the waves to return to the sender is shortened, so the sensor detects motion and trips a light or alarm switch, if so wired and programmed. The continuous-wave, radar motion detectors work on this same principal, however, this system uses radar waves in place of the ultrasonic sound waves.
Much like a police radar gun uses radar waves to detect the rate of time it takes a radar wave to reach a target and return to the gun, the presence of motion is also detected. The radar gun measures the changing time it is taking the waves to be sent out and returned and then calculates the rate of speed by the changing times. The motion detector needs only to sense the change in time for a single radar wave to return to detect motion. Vibration sensing motion detectors commonly use a network of sensors placed throughout an area to measure any vibration on or in the ground as detected motion.
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