From automatic doors at the store to burglar alarms in homes, motion detectors are present throughout our daily lives.

Rarely does one have the time to consider how an automatic door “knows” when to open or what causes a security alarm to sound.

The answer is motion detectors, simple devices that exploit clever physical principles.

Motion sensors can be grouped into two categories­—passive and active. Passive detectors functions by detecting infrared energy, while active ones put energy into their surrounding environments to detect motion.

All motion detectors have a normal state where the system is in a stable or optimal environment, which means there is no unexpected movement being detected. This explains why automatic doors will not open when a paper bag flutters by.

Automatic doors are active systems. They use radio emitters and detectors set up near the door frame to detect motion. The emitter sends out microwave radiation which is picked up by the detectors.

When a person walks towards the door, it causes an interruption in the flow of microwave radiation from the source. The box on the door frame senses this delay and opens the door.

Burglar alarms, on the other hand, are passive motion detectors. They detect changes in infrared, or heat energy, in their surroundings. Since people are warm-blooded, we radiate heat all the time, and as someone moves past the detector, it senses a rapid change in the infrared energy it receives and sets off the alarm.