In the 1800′s, a German physicist sat at his work table, studying the effect of radio waves on different materials. Suddenly, he had an epiphany. He discovered that some of the waves were able to pass through certain materials, while some materials reflected back the radio waves. He quickly collected data on this research, which led to the development of radio, and eventually became the foundation for radar technology.
This physicist was named Heinrich Hertz, and he is the man that made today’s motion detecting technology possible. Even though effective Radar systems were not successfully developed until the 1940′s (during World War II), Hertz’s research was essential. By using radar, the Allied forces were able to detect enemy craft before they attacked, saving the lives of many soldiers.
Today’s motion detecting technology uses various combinations of researched motion detecting methods. Here is a guide to five of the most common types of motion detectors.
Active Infrared Motion Detector (IR)
Active infrared motion detectors use an IR sensor, as well as a source of radiation. The sensor is able to detect interruptions in the radiation it receives from the radiation source. This basically means that an IR motion detector is able to detect the signal of heat energy emitted by an intruder as it differs from the constant infrared scanning activity of the detector, as long as the intruder passes through it’s detection range. [Editor's note: I believe this paragraph is more accurate now given John's comment below. Thank you John!]
Dual PIR Sensor Coverage Area
Passive Infrared Motion Detector (PIR)
PIR (or Passive Infrared) motion detectors are currently the most popular type of motion detecting system. PIR motion detectors are relatively inexpensive compared to detectors that utilize other technology. A PIR sensor is able to use an optical collection system in combination with several different sensing elements. This allows the sensor to detect any changes that occur in ambient infrared radiation. The sensor is able to recognize a thermal infrared image (created by infrared technology combined with a heat sensor), which usually starts the transmission of an alarm signal back to the unit’s receiver. If the PIR sensor is connected to a monitored security system control panel, the control panel can interpret the alarm signal, and convert it into data to trigger the appropriate alarm.
Continuous Wave Radar Motion Detector (CW)
Continuous Wave (CW) motion detectors use microwave signals to emit frequencies to bounce off of the surrounding area (which is why they are sometimes referred to as “microwave motion detectors”). The sensor is able to detect when there are subtle changes in these frequencies, since this would signal a disruption. When an intruder passes the field of a CW microwave sensor, he disrupts the frequency, which sets off the sensor’s alarm. CW motion detectors are generally more expensive than PIR motion detectors. This is because they are highly sensitive, and are very reliable over longer distances than other sensors.
Ultrasonic Motion Detector
An ultrasonic motion detector is able to use sound energy in order to detect movement in a specific region. This ultrasonic sound energy is emitted in waves, which come from quartz-crystal transducers. When the sensor detects movement, the sound waves are disrupted, which triggers the sensor. Ultrasonic motion detectors are commonly used in automatic doors, since they are reliable and easy to install. The main limitation of an ultrasonic motion detector is that it can be blocked by any material, and can be easily disrupted.
Vibration Motion Detector
A motion detector that detects simple vibration can either be made from materials at home, or purchased as an electrical device. Most vibration motion sensors use the peizoelectric effect (the ability of some materials to generate an electric field) in order to detect motion. Simple do-it-yourself vibration sensors most commonly use a lever that activates a switch when it detects vibration.
Sources:
http://www.electronics-manufacturers.com/info/sensors-and-detectors/motion-detector.html
http://www.fi.edu/weather/radar/history.html
http://en.wikipedia.org/wiki/Piezoelectric_sensor
http://www.alarmsystemreviews.com/motion-detector-microwave-infrared-info.html
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