Home Insurance Guide

The last article about how door and window sensors worked probably piqued your interest in the other sensor your security system uses, the motion detection sensor. Whereas the door and window sensors monitor ingress and egress, the motion sensor monitors what happens when the bad guys get inside - especially valuable if they defeat the door/window sensor. It happily sits in the corner of the room several feet up on the wall and blinks red or green or blue or whatever, keeping its watchful eye on the room and checking for any motion. So, how does it actually detect motion?

Motion sensors come in two main varieties: radar-based and photo-sensor. The radar-based is exactly what it sounds like, it’s a radar for the inside of your home. It sends out microwave radiation bursts and remembers the reflected pattern in the room. If this pattern changes, it signals the control box that the zone has been compromised. This is how automatic doors work at grocery stores and the like, that’s why they’ll trip off your car’s radar detector.

The photo-sensor is sort of like the laser beams in museums in movies where a beam of light is emitted from a source and expected by a receiver. If the beam of light is broken, then the sensor notifies the control box that the zone has been compromised.

Another type, less often used because it’s more expensive, is a passive infrared motion detector that scans the room for infrared energy. If a human being, with generates lots of infrared energy, walks in range of the sensor, it will detect it and notify the control box.