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What Is an Ultrasonic Motion Detector?

By Kurt Inman
Updated May 16, 2024
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An ultrasonic motion detector is a device that can detect movement of people or objects within a limited area. It senses motion by analyzing sound waves in its environment. Some just listen for sounds, while others send out ultrasonic signals and analyze how they are reflected back. This type of motion sensor is often used in a home security system to sound an alarm if anything moves. It can also be used to turn on lights or other devices when someone enters a room.

Active motion sensors generate sound waves in the ultrasonic frequency range, typically around 30 to 50 kilohertz (kHz). These frequencies are generally inaudible to humans and most animals and do not pass through most objects. The ultrasonic motion detector emits cone-shaped sound waves using a quartz transducer and listens to the echoes. When no objects are moving in the area, the pattern of sound and the time it takes to bounce back remain the same. If something moves, the detector senses that the level or phase of the returning sound waves has shifted slightly.

An active motion detector should try to eliminate as much noise or false data as possible. To do so, it often analyzes the Doppler Effect frequency variations in echoes, as well as amplitude changes. This type of detector is fairly complex and often includes a programmable microcontroller. In contrast, a passive motion sensor listens for suspect sounds like metal scraping or breaking glass. Unlike an active sensor, it does not generate any audio waves on its own.

Background noise and motion can often unintentionally trigger an ultrasonic motion detector. Birds, insects and wind gusts make it difficult to use one as an outdoor motion sensor. A low-cost active detector, such as a typical car security sensor, may only analyze variations in echo amplitude. Ignoring frequency variations can result in false alarms from this type of detector. Ringing telephones and other loud noises sometimes inadvertently trigger passive motion sensors.

Furniture, boxes and other large items can "hide" other objects in the area from an active ultrasonic motion detector. They can also absorb and distort sounds that are critical to a passive sensor. Breezes from air conditioners or open windows are often sensed as movement, too. Some motors and fans generate sounds in the ultrasonic range, which may also confuse motion sensors.

Using an ultrasonic motion detector that includes several active and passive sensors can improve results. Some systems combine an ultrasonic sensor with a passive infrared motion detector. Analyzing the sound and heat data together often gives a more accurate picture than either type would on its own.

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