Motion Sensors for Home Security
Motion sensors are devices that can be used to trigger an action when motion is detected by the sensor. An electronic motion detector contains a sensor that transmits a signal when triggered by motion. That signal can be used to activate an alarm, a light, or practically anything electrical or electronic.
Motion detectors primarily use one or more of three technologies; Ultrasonic, Microwave and Passive Infrared (PIR).
Ultrasonic sensors (active) send out pulses of sound (beyond the range of human hearing) and measure the reflected waves. When an object moves, the reflected wave changes and can trigger the detector. The sensitivity of the device can be modulated somewhat, however they are prone to false alarms. The sensitivity declines with distance, their range max's out at around 40 feet. Passive sensors simply listen for disturbances like breaking glass. Passive sensors can be very sensitive and tend to be more expensive. Although, like active sensors, they are also prone to false alarms, such as from the sound of a ringing phone. Models designed to ignore certain sounds can also miss sounds that should trigger an alarm.
Microwave sensors also send out pulses but in the microwave band. They are not harmful to people. Microwave sensors are more sensitive, especially for forward motion and have a range of about 150 feet. Microwave sensitivity can be a problem as the waves can penetrate walls where motion would be expected. This type of motion sensor also draws more energy than the ultrasonic or PIR detection.
Passive Infrared (PIR) sensors measure infrared light, i.e. heat. The sensor can trigger an alarm when a heat level changes in intensity or position. This type of sensor is generally less expensive and draws less power. One drawback is that its field of view is not as broad as other sensors; ceiling mounting can help reduce this weakness. Another shortcoming is that motion directly toward the sensor may fail to result in a trigger. This type of sensor should not be mounted near ventilation ducts, pointed toward windows or placed where temperature changes will be common.
Dual technology devices use a combination of sensors to improve detection and reduce false alarms. PIR and ultrasonic combination sensors are the most common because they compliment the other by filling in where the other has a weakness. Such devices are more practical because they are designed to ignore small changes, like a fan's oscillation or the movement of a pet.
Motion sensors are commonly used to trigger alarm systems. They also are used for outdoor security lights, the type that come on when something moves in your yard or driveway. They can also be used to trigger an alert, such as a door opening or car entering your driveway. Occupancy sensors couple a motion sensor with a timer. The circuit is on when motion is detected and for a set period time following. If no additional motion is detected within that time period, the circuit is turned off. Such occupancy devices are commonly found in public restrooms.
When selecting a motion detection sensor, those devices using "dual technology" are better than devices using a single sensor. Because similar devices can be tailored to specific tasks, select a sensor that is tailored to your task. Make sure that it has the range and field area that suits your needs.