A motion detector is a device that contains a physical mechanism or electronic sensor that quantifies motion that can be either integrated with or connected to other devices that alert the user of the presence of a moving object within the field of view. They form a vital component of comprehensive security systems, for both homes and businesses.
There are basically three types of sensors used in motion detectors spectrum:
Passive infrared sensors (PIR)
Looks for body heat. No energy is emitted from the sensor.
sends out pulses and measures the reflection off a moving object.
Sensor sends out microwave pulses and measures the reflection off a moving object. Similar to a police radar gun.