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What is a Motion Detector?

R. Kayne
R. Kayne

A motion detector is a device that monitors a field of view and performs a function if motion is detected within that field. The function might be to trigger the opening of a door, as in the case of a grocery store; start a videotape machine for surveillance; turn on floodlights; or sound an alarm. A motion detector might detect motion through the use of optics or acoustics and can be passive or active.

Passive infrared (PIR) motion detectors are commonly used inside homes, linked to security systems. The unaided human eye cannot see infrared light, but the human body generates an infrared signature in the form of heat. A PIR does not emit signals, but monitors the temperature of the field of view looking for changes in the infrared spectrum. A human body moving through the field can trigger the device to sound an alarm, call a monitoring agency, or perform another function.

Human ears cannot detect ultrasonic waves.
Human ears cannot detect ultrasonic waves.

An active motion detector emits optics or sound waves and measures feedback to detect motion. The simplest type is commonly used in commercial doorways to trigger a doorbell. A device is fixed to one side of the doorway, an optical sensor to the other. A beam of light passes from the device to the sensor. When someone enters the establishment, the beam is broken, triggering the doorbell.

Another kind of active motion detector is used at grocery stores to automatically open the doors for customers. This emits radar pulses; monitoring the time it takes the energy to bounce back. When any object disrupts the radar’s reflection, the detector triggers a device that opens the doors.

Other active motion detectors emit ultrasonic acoustic waves to detect motion. Any object moving across that plane will disturb the acoustic signature and change the picture. The human ear cannot detect ultrasonic waves, but certain animals are sensitive to ultrasonic signals.

Some types of motion detectors turn on floodlights in the yard, driveway or porch when motion is detected. After a period of time with no movement, the lights go out. Some motion detectors can be set to be less sensitive to the movement of small animals.

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Discussion Comments


@Grivusangel -- That would be helpful, wouldn't it? We need to replace or reposition the motion light outside our house. It's too sensitive and comes on if one of the neighborhood cats ambles by.

That can be frustrating, not to mention being a hindrance to a good night's sleep! So I think it's time to do something about that motion detector.


The outside light at the side door of our church is supposed to be a motion detector, but it only works when it feels like it. You just about have to wave your hand in front of it to get it to turn on. Very frustrating, considering it's a safety measure!

I think it's about time we replaced the light with one that actually comes on when someone walks in front of it or opens the side door!

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    • Human ears cannot detect ultrasonic waves.
      By: Den
      Human ears cannot detect ultrasonic waves.