Motions sensors work by implementing one of two types of systems to detect motion. These systems are called passive sensors and active sensors. Passive sensors examine the area around the device to determine activity. Active sensors send out one of three types of energy to identify movement in an area.
A common type of passive motion sensor is a passive infrared sensor that can detect heat. This system works by monitoring a room for any significant change in temperature such as from a person's body temperature.
Active sensors detect motion using one of three different types of energy. These types are infrared light, microwave radiation or sound waves. Microwave radiation motion sensors send out microwave signals that bounce off of objects to detect movement in an area. Area reflective motion sensors are similar to microwave radiation motion sensors but instead of sending out microwave pulses, infrared rays detect motion. Ultrasonic motion sensors work by using sound signals played into an area. These signals bounce off of moving objects to detect motion.
Dual technology motion sensors implement more than one type of motion sensing system to detect motion more accurately. These types of motion sensors use two systems, such as one passive and one active motion sensing system, in tandem to limit the amount of false alarms. This is because motion must trigger both types of sensors for the alarm to sound.