Motion detectors are an important part of any well-designed business or home security system. The best home security systems use a variety of techniques and technologies to detect an unwanted intruder, and motion detection is an additional layer of security that greatly enhances the efficacy of any home security system, without greatly increasing its price.

Although the technology is constantly advancing, and there are any number of niche solutions that rely on a variety of underlying theoretical models, motion detectors can be broadly grouped into three different classifications:

Passive Infrared motion detectors work by detecting the heat of a human body. This a highly effective detection system but it can generate false positives, whether from a pet, sunshine streaming in on a particularly hot day, a space heater activating when its thermostat drops below a certain point, and so on. However, passive infrared systems can be tuned to avoid false positives by requiring a certain magnitude of heat change before the alarm is triggered. For example, a sensor can be tuned to not go off when a pet enters the room, by excluding heat signatures below a certain size.

Ultrasonic motion detectors work by bouncing ultrasonic waves around a room or space, and detecting changes in the contents of the space by changes in the reflected waves that return to the sensor. This type of sensor is difficult to defeat, but is also difficult to program to ignore pet entry into a home area, making it better suited for areas where there is no legitimate traffic which the system must ignore.

Microwave motion detectors bounce microwaves around the area, much like the radar guns used by the traffic police. Like ultrasonic systems, they are effective and hard to defeat, but also hard to customize.

Image recognition motion detectors work by assigning a computer to continually process the input from a digital video camera. The computer algorithm constantly scans the pixels of the transmitted image, and triggers an alert when enough pixels change in value, indicating an intruder or major change to the monitored space. This is a high-end solution.

Laser motion detectors send a laser beam (or a photoelectric light beam) across an area and detect interruptions to the beam from a sensor located across the space from the beam source. These forms of detectors are somewhat out of date, because they are relatively easy to evade if an intruder knows of their presence.

A motion detector can be wired into the business or home security system and programmed to perform a wide range of tasks in response to an intrusion. The system can call a designated phone number with an alert message, send a signal to a monitoring office (which can then dispatch an investigative team), turn on a light, sound an audible or silent alarm in your home security system.