Motion sensors
everywhere
When you first run across a motion sensor, it seems like magic. Lights come on without anyone flipping a switch. Doors open without you turning a knob. Motion detectors are so commonplace nowadays that people often take the technology for granted. You may not have realized, however, that motion sensors can use radar or infrared technology to do their work.
How It Works
Motion sensors come in two basic varieties-active or passive. The active radar sensor sends out electromagnetic energy and detects the returning echo much like a bat. The sensor detects the echo and determines whether a change has been made like something coming close to it. If a change has been made, the sensor can activate whatever device is attached to it. The passive infrared sensor does not send out energy; it detects energy in objects near it like body heat. If the sensor detects a fast change, the sensor triggers the device attached to it.
World War II
Though the study of what would become radar began in the late-19th century, it was World War II that refined it to a useful detection technology. The use of radar made it possible to detect aircraft--and eventually undersea aggressors--before they could become dangerous to targets. This early use of radar was strictly confined to military applications.
Nonmilitary Uses
As the war wound down, Samuel Bagno used his knowledge of radar to develop the first motion sensors in the mid-1940s. He called his invention an ultrasonic alarm. The device sent ultrasonic waves throughout a room. When something disrupted the waves, a return echo triggered the alarm. This first success of a nonmilitary application of radar created a commercial demand for more ways to harness radar technology.
Alarms
The 1970s saw motion sensors integrated into alarm systems. These alarm systems continued to use the principles of Bagno's ultrasonic technology. A sensor emitted an ultrasonic signal and detected changes in the echo. If a change was detected, the sensor notified the alarm system's control panel. The problem was that false alarms were common. The simplest things like a phone ringing or clock chiming could change the ultrasonic wave's echo enough that the alarm sounded.
Modern Sensors
Infrared motion sensor began replacing radar sensor in the 1980s. Initially, the devices were expensive, but as their uses expanded, the prices became more attractive; in turn, the use of infrared sensors compounded. Today, these sensors are used not only in alarm systems but on floodlights, public toilets, paper towel dispensers, doors, room lights and more.
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