Motion Detectors - Passive Infrared (PIR) motion sensor technology provides one of the most reliable and cost effective methods for protecting spaces in the home or office and is the most widely used for security applications.

Other forms of motion detection include microwave and sonic waves that can detect a 'doppler' shift in a moving object.

Infrared Motion detection is a passive system that detects infrared energy. These sensors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that can detect a human being, you need to make the sensor sensitive to the temperature of a human body. Humans, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers.

Motion detectors are sensitive to motion, but not to a person who is standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy it is seeing. When a person walks by, the amount of infrared energy in the field of view changes rapidly and is easily detected. You do not want the sensor detecting slower changes, like the sidewalk cooling off at night.