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How Does a Motion Detector Work?

Staff Writer, Safety & Security

March 15, 2023

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Motion detectors are essential home security system components that alert you if someone is prowling in or around your home. Though there are a few different types of motion detectors, most rely on similar technologies to detect intruders: they pick up on tiny changes—in things like temperature, vibration, or radiation—in the spaces they cover.

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How do motion sensors work?

There are several types of motion sensor technology—active ultrasonic and passive infrared (PIR) are two popular options, though microwave, tomographic, and dual-technology detectors also get some market share.

An active ultrasonic motion detector emits ultrasonic sound waves that reflect off objects and bounce back to the original emission point. When a moving object disrupts the waves, the sensor triggers and completes the desired action, whether this is switching on a light or sounding an alarm.

Motion detection cameras, lights, and sensors used in home security systems generally rely on PIR sensors. These detect infrared energy, which humans and animals release as heat. If the motion sensor detects an increase in infrared energy, meaning someone has approached the sensor or walked within its range, it notifies the home security control panel and an alarm sounds.

PIR sensors can be set to ignore small changes in infrared energy, so they won’t trigger an alarm if the family cat or dog walks by or if the household temperature fluctuates slightly.

Some other sensors use microwave or tomographic technologies, both of which emit waves of radiation—microwave and electromagnetic, respectively. When someone or something disrupts the radiation waves, the sensor sounds an alarm.

Dual-technology detectors are, as their name implies, sensors that rely on two different motion-detecting technologies to watch for movement. These are a bit more impervious to false alarms, as both technologies have to be tripped for the alarm to sound.

Examples of motion detectors

Several types of security devices use motion sensors:

You can use motion sensors to automatically turn on a light when someone approaches your home, sound an alarm when movement is detected in your living room, or trigger a motion sensor camera to start recording when someone enters your home.

Installing motion detectors

As motion sensors have a limited range—anywhere from 50 to 80 feet—you’ll want to place several around your home, especially in areas where people frequently walk, such as hallways, staircases, living rooms, and bedrooms. As a motion sensor may have a harder time detecting someone walking directly toward it, it’s better to place the device alongside a hallway where a burglar would walk parallel to it.

When deciding where to place each motion sensor, make sure they are at least 10 feet away from bright windows, radiators, and heating vents, as abrupt changes in temperature or HVAC-related airflow may trigger false alarms.

Compare the best home security systems

Brand	Best for	Monthly cost	Starter equipment price	Contract required	Smart home compatibility	Learn more	Read review
Vivint	Best overall	$29.99/mo.	$599.00	No *	Amazon, Google, Z-Wave	View packages	Read review
SimpliSafe	Budget pick	$17.99/mo.	$245.00	No	Amazon, Google	View packages	Read review
ADT	Most experienced	$28.99/mo.	$0.00	Yes	Amazon, Google, Z-Wave	View packages	Read review
Cove	Best customer experience	$17.99/mo.	Quote required	No	Amazon, Google	View packages	Read review
ADT Self Setup	Best optional monitoring	$19.99/mo.	$199.15	No	Amazon, Google, Z-Wave	View packages	Read review

Info current as of post date. Offers and availability may vary by location and are subject to change.
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Written by

Cathy Habas

With over eight years of experience as a content writer, Cathy has a knack for untangling complex information. Her natural curiosity and ability to empathize help Cathy offer insightful, friendly advice. She believes in empowering readers who may not feel confident about a purchase, project, or topic. Cathy earned her Bachelor of Arts degree in English from Indiana University Southeast and began her professional writing career immediately after graduation. She is a certified Safe Sleep Ambassador and has contributed to sites like Safety.com, Reviews.com, Hunker, and Thumbtack. Cathy’s pride and joy is her Appaloosa “Chacos.” She also likes to crochet while watching stand-up comedy specials on Netflix.