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How does a Motion Sensor Work?

By G. Wiesen
Updated: May 16, 2024

The way in which a motion sensor works typically depends on the type of sensor being used, which often depends on the device that uses the sensor. One of the most common types of sensor technology is an active sensor that sends out bursts of energy, which bounces back in a way similar to sonar. There are also passive sensors that do not send out any type of signal, but instead receive energy from their surroundings to detect motion. Some systems can also use a combination of both active and passive technology to create a motion sensor that emits and receives an energy signal.

A motion sensor is an electronic device — often part of a larger system, such as a light or camera — that detects motion to activate the system. There are two basic types of motion sensors: active sensors and passive sensors. Active sensors emit a signal, typically an ultrasonic burst of sound waves, similar to a sonar system, which is reflected by the surroundings; this reflected signal is received by the sensor. When something moves within the area of an active motion sensor, the change in signal that is reflected to the sensor activates the system. This type of sensor is often used for indoor security and automatic garage door openers.

Passive sensors are a type of motion sensor that do not emit a signal, but instead detect infrared radiation around the sensor. When a person or animal moves through the area, heat from the movement is detected by the sensor, which then activates the system to which it is connected. This type of motion sensor is often used for motion-activated lights or cameras in a security system. Passive sensors are typically set to detect only sudden or extreme changes in thermal radiation, which prevents such systems from activating due to changes in surrounding temperature caused by the sun rising or pavement cooling at night.

A motion sensor can also utilize a combination of passive and active technology. This type of system is often used in stores to trigger a sound when someone enters the business. The motion sensor usually consists of two parts: one that emits a laser or similar energy signal, and a second piece that receives the signal. When someone passes through the system, the signal is interrupted and the receiving piece activates a response such as a bell. This type of system is not usually used for security purposes, since the energy beam could be avoided quite easily to keep from activating the system.

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Discussion Comments
By anon944838 — On Apr 09, 2014

We have a security system installed in our home. Recently we bought a time activated gadget that turns our living room light on at a certain time of the evening. When the light turns on, will it activate the motion sensor when we are away?

By LisaLou — On Jul 27, 2012

We have a motion sensor camera set up in our timber during hunting season. Once the camera senses motion, it will take a picture. We bought this in hopes of catching some great shots of deer.

While the camera has taken some shots of deer, most of them are of smaller animals that we aren't interested in getting pictures of. It will take a picture of anything such as a squirrel or rabbit than runs in front of it.

In the fall when the leaves are falling from the trees, sometimes this will even activate the sensor in the camera. The concept behind these cameras is great, but I wish there was a better way to control what it actually takes pictures of.

By honeybees — On Jul 26, 2012

We have a small garage and too many things to try and fit in this space. Because of this our garage is always cluttered with stuff. If something is in the way of the garage door sensor, the door will not close properly.

The door will start to come down, but as soon as it gets to a certain spot, the sensor is activated and the door will automatically go back up.

This is good as far as a security measure, but there have also been times when I left the house thinking I closed the garage door and came home and found the door wide open. If I cleaned out my garage and didn't leave anything close to the sensor, I wouldn't have a problem with this.

By SarahSon — On Jul 25, 2012

I don't totally understand how a motion sensor light works, but I am glad they do. We have these outside our house and the one that I benefit from most of the time is right by the back door.

Whenever it is dark outside and I need to see to unlock my door, the motion sensor light automatically comes on. I don't have to fumble around in the dark anymore looking for a way to unlock the door.

We also have these at the front door so as people walk by, the front steps are automatically lit up for them. This saves on energy since the lights only come on when there is motion and don't stay on all the time.

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