Motion Detectors
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Description
As the name implies, a motion detector is made to detect the motion of living objects. There are four different types of motion detectors, with variations in each type.
PIR
The first and most common type of motion detector is called a Passive Infra-Red motion detector, or PIR. As indicated by the word "passive", a PIR does not send out any type of signal. It simply looks at the amount of heat in an area and detects movement by tracking the changes in heat. The detector divides the area of coverage into several beams, and detects changes between the beams. For this reason, PIR units are more sensitive to movement across the coverage area, than they are for movement forward to and away from the detector. Some PIRs have an adjustable "pulse count" feature, were each trigger by the unit is a "pulse". By varying the number of pulses needed in a given interval to trigger the alarm, the sensitivity of the unit can be adjusted.
There are several variations of the basic PIR. Cheaper PIRs use analog technology, while more expensive units are digital. The digital units process the raw signals coming in from the IR receiver, allowing greater discrimination and sensitivity. Some "pet immune" units are pre-calibrated to allow up to a certain amount of "pet" (i.e. a 80 lb pet immune unit would allow the amount of heat generated by a 80 pound dog), before it would register movement. "Pet alley" units are setup so they do not detect movement below X feet above the floor. Another version of the basic PIR is the Request To Enter (RTE) unit. These are very high sensitivity units placed above doors. When the RTE unit detects movement, it triggers the door-opening unit.
There are several problems with PIRs. With the exception of a few units specially made for outside mounting, PIRs should not be located in direct sunlight. A cloud passing in front of the sun can set off incorrectly installed PIRs. Likewise, they should not be installed were the output from a forced hot air system is in the sensors field of view. PIRs loose sensitivity when the ambient temperature approaches that of the human body. Finally, as any cat owner can tell you, pets do not always stay on the floor, and can cause false alarms when climbing on furniture.
The benefits of PIRs are low cost (in comparison to some of the other types of motion detectors), and (relatively) easy installation.
Microwave
Another type of motion detector is the microwave unit. Unlike the PIR, the microwave unit sends out a signal, and then listens for the echo when it bounces off an object. It is very similar to a miniature radar unit. The detector looks for changes in the echo pattern to detect movement. As with PIRs, higher cost microwave units have greater processing capabilities, with the related increase in sensitivity and decrease in false alarms. Some older units used ultrasonic sound waves instead of microwaves, but the ultrasonic units have fallen out of favor because of false alarm problems.
Microwave units are more accurate then PIRs and less likely to create false alarms. The downside of microwave units is a higher cost, and more difficult installation.
Dual Tech
A dual tech motion detector is a unit with both a PIR and a microwave mounted in a single case. They can be set to trip if either detector senses movement, or to only trip when both detectors sense movement. As expected, dual tech units are more expensive than individual microwave or PIR units.
Photo Beam
Photo beam AKA photobeam AKA infrared beam units use a beam of infrared light that is sent from a transmitter to a receiver. When someone breaks the beam, it triggers the alarm. Sometimes both transmitter and receiver are mounted in the same unit, using a mirror to return the beam back to it's source. A common use for photobeam units is the "bong" to announce when someone enters a store in a mall. The beam on photobeam units can be avoided by going above or below the beam. Also, anything that breaks the beam, i.e. a leaf, fog, pets, etc, will trigger the alarm. Because of these problems, photobeam units are only used in a few specialized applications.
