An optical, microwave, or acoustic sensor is used by a motion sensor alarm to identify motion in and around your home or place of business. Its typical application in physical security is to find intruders inside a building and sound an alarm.

A motion sensor alarm system is made up of a few major parts. The most visible component of it is a motion detector, which is why the entire motion sensor alarm system is occasionally referred to as just a motion detector. This motion detector typically consists of an element that generates infrared, microwave, or ultrasonic waves, as well as a receiver that picks up these waves or radiations.