What is a Microwave Motion Detector?

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  • Written By: A. Rohlandt
  • Edited By: Jenn Walker
  • Last Modified Date: 06 November 2019
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A microwave motion detector is a device that uses Doppler radar to detect and measure the movement of an object. For most people, the two most familiar forms of this mechanism are used in a home security system and in traffic law enforcement. Another familiar application is the use of a microwave motion detector to operate lighting devices upon a person entering or leaving an area.

The microwave motion detector typically is a single unit device that emits a pulse of microwave energy and then detects its reflection. Calculating the time required for the reflection and the change in frequency, it determines the distance, direction, and speed of the object being observed. Direction and speed are computed based on the Doppler effect. Most experience the Doppler effect when hearing a train or other vehicle approaching them; the sound seems to move higher in pitch as the object approaches and lower as it moves away.

In security applications, the microwave motion detector is usually paired with an infrared motion detector. This is to reduce the possibility of false alarms. The microwave motion detector may detect movement of an inanimate object behind a wall or door, such as a windblown tree limb, for example. If the infrared sensor also detects body heat, the likelihood that the object being detected is an intruder is increased, and the security system can execute the required function.


In traffic control and law enforcement, the radar gun is the most familiar type of microwave motion detector. Permanently fixed detectors have the capacity to monitor speed and direction of vehicles and can be used to trigger cameras; traffic signals; or other control devices, such as gates. Some vehicles employ a car motion detector for collision avoidance in addition to providing anti-theft features.

Manufacturing or industrial applications make use of motion sensors as a type of control in production and packaging operations. Sensors can alert operators of a change in assembly speed or the staging of the finished product. Automated material handling devices also employ sensors in computer-controlled warehousing.

A microwave motion detector also is often employed in larger commercial buildings as a tool in energy management. Turning on lights when a person enters a room and then turning them off after a period of time if the person leaves or stops moving has proven to be quite effective in reducing operation costs. Sensors can also serve in a security fashion — not as theft prevention devices, but as protection from slips and falls due to poor lighting conditions.

Used alone, microwave motion detectors have some drawbacks. They are capable of detecting movement behind non-metallic objects, raising the incidence of false alarms. They may also be fooled by slow lateral movement relative to the sensor. Another shortfall is power consumption; they are typically programmed to signal at intervals to reduce power use, and this could allow an intruder to avoid detection.


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