Motion Detectors: Feeling the Heat

One part of the security system that lets homeowners rest easy is the motion detector. Read on to learn more about how these silent sensors work.

If a burglar enters a security-system-protected home, he’ll likely step right into a minefield of invisible tripwires that will send the cops speeding his way. The technology that makes this possible is the motion detector—specifically, the PID, or passive-infrared detector. “Passive” means that the device produces no infrared-heat energy itself, which allows for multiple detectors to work simultaneously without triggering each other’s alarms. These devices don’t simply detect infrared energy itself, which is constantly emitted by the sun and from the surface of the earth. Because most homeowners don’t want their alarms triggered by a tap-dancing centipede, PIDs are only programed to react to large, sudden changes in infrared energy.

Positioned behind a lens, a pyroelectric sensor inside the PID takes constant readings of infrared energy via interior reflectors that give the device a “view” of the room. This sensor allows the PID to monitor the room’s ambient energy levels so that it doesn’t react to gradual changes in temperature. An abrupt change, however—like that of an intruder brandishing his 98.6-degree body temperature—will trigger the PID and trip the alarm.

Although modern PIDs are less likely to go off at the drop of a microwaved hat, a few things can still trigger false alarms. These include HVAC vents, sudden drafts, and, occasionally, pets—although careful positioning and special pet-immune PIDs should solve this problem for all but the largest or most acrobatic critters. And despite the fact that most PIDs do not detect well through glass, homeowners should avoid pointing detectors towards windows. For those areas, glass-break sensors will be much more likely to catch intruders.