5 Types of Motion Detectors and How They Work
Motion sensors can use a few different technologies to detect movement. Each one comes with its own strengths and weaknesses, so choosing the right one for you shouldn’t feel like “Sophie’s Choice.” To help you figure out which type of motion detector best fits your security needs, we broke down the five technologies you’ll see in home security motion detectors.
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Did You Know? The Department of Homeland Security recommends using “multiple detection technologies to protect against the weaknesses of any one technology” in its handbook for Intrusion Detection Sensors.2
Active Ultrasonic Motion Detectors
Ultrasonic motion sensors map out their surroundings in the same way as bats. They emit ultrasonic sound waves, and then detect the reflection. When the waves hit a stationary object, they bounce back to the sensor with the same frequency the sensor emitted.
When a moving object encounters the waves, it changes the frequency of the reflected waves. The moving object either absorbs some of the wave’s energy or adds energy to the wave, which changes the wave’s frequency. The change in frequency, known as the Doppler effect, triggers the detector.3
What makes these motion detectors active? Since they need to emit an ultrasonic sound wave, they require an active source of energy to generate the sound wave they emit.
Active ultrasonic motion detectors work great for covering large areas with obstructions. Since sound can reflect around corners, the sensors can still detect movement around corners, minimizing the impact of couches, pillars, doorways, and more. They’re prone to false alarms from any moving object in the motion-detection area, however, such as a fan switching on or curtains fluttering in a breeze.
Passive Infrared (PIR) Motion Detectors
Instead of actively sending out signals, PIR motion detectors passively monitor the environment for changes in infrared energy levels. That sounds technical, but all it means is that they can sense body heat. So any warm-blooded animal can trigger a PIR motion detector.
It’s great that PIR sensors give you a way to detect motion without an energy source, but the sensors require careful placement. Any electronic device or piece of equipment that generates heat can cause false alarms. A burglar can also mask their body heat to bypass the sensors without much effort.
Microwave Motion Detectors
No, it can’t heat up your food. But these motion detectors use the same type of energy as microwave ovens, except in much smaller amounts. They actively emit microwaves and monitor their reflection using the Doppler effect to detect movement, just like ultrasonic motion detectors.
The great thing about microwaves for motion detection is they can penetrate most walls. That lets a single motion detector cover multiple rooms or rooms with a lot of obstructions, like your living room. It also, however, makes them more prone to interference and false alarms from other electronic devices. They also consume more energy than ultrasonic sound waves, which means the motion sensor takes up a bit more space in the device.
That said, their ability to “see” through walls like Deadpool sees through the fourth wall makes them well worth it for spaces with physical obstructions.
Tomographic Motion Detectors
Instead of using one sensor to detect motion in a fan-shaped area, tomographic motion detectors create a mesh network of motion-detecting nodes. Each node uses radio waves that bounce around the monitored area, filling the environment. When anything in the mesh network moves, it disrupts the radio waves, which triggers an alarm.
Tomographic detectors work great for covering large spaces, which makes them perfect for large, open homes. But creating a tomographic mesh network costs quite a bit more than a single motion detector, so it’s a premium option.
Dual-Technology Motion Detectors
You can probably tell what this one is from its name, but I’ll explain it anyway. Dual-technology motion detectors use two different types of motion sensors in a single device. Most use a PIR motion sensor and an active sensor to minimize power requirements. They also reduce the chance of false alarms, since both sensors need to detect motion before triggering an alarm.
With dual-technology detectors, intruders need to evade detection by only one of the sensors to evade detection entirely. Basically, they give potential intruders more options for skirting your security measures. They’re a great option for supplementing your security system, but you don’t want to rely only on dual-tech motion detectors.
>> Read About: 10 Ways to Stop Criminals From Choosing Your Home
