The Types of Motion Detectors

Active Infrared Motion Detectors

• Active infrared (IR) motion detectors operate in conjunction with a radiation source. They are able to sense disruptions in the radiation that they receive, usually caused by an intruder's body heat. In order to sense an intruder, therefore, the intruder must pass through the field generated by the radiation source.

Passive Infrared Motion Detectors

• Passive infrared (PIR) motion detectors operate from the same general principle as active infrared motion detectors, but they do not require a radiation source. Instead, they are able to sense changes in the surrounding area's ambient infrared radiation. Usually, PIR motion detectors have several different motion-sensing components, such as both an infrared sensor and a heat sensor. PIR detectors are the most widely used type of motion sensor.

Continuous Wave Radar Motion Detectors

• Continuous wave radar (CW) motion detectors work through the use of microwave signals. These signals emit frequencies that bounce off of the objects in the surrounding area. The detector is then able to track disturbances in these frequencies. These detectors are more sensitive than PIR detectors, and therefore usually cost more. These types of detectors usually use microwave oscillators, detector diodes and audio-frequency processing electronics to accomplish this task. They work on the same basic principle as military radars.

Ultrasonic Motion Detectors

• Ultrasonic motion detectors operate by tracking changes in sound frequency. The detector emits a sound frequency in waves that sweep across the space that the detector is monitoring. When the wave encounters an object, its frequency is disrupted and the alarm in set off. These detectors are used in automatic doors and security alarms due to their ease of installation and reliability.

Vibration Sensors

• Vibration sensors use piezoelectric technology in order to detect the movements of objects around them. The piezoelectric effect was discovered by Jacques and Pierre Curie in the 1800s. They found that certain materials have the ability to change mechanical pressure into electrical output. In other words, they generate an electrical field.