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Motion Detectors
SEM Security Systems
a division of Southeast Mobility Inc
Motion detectors are the most widely used sensors in the industry, they are also the sensor that is most responsible for false alarms. For this reason it is very important that care be taken in the location and mounting of motion detectors.

There are basically two technologies that are used in motion detectors, Passive Infrared (PIR) and Microwave. The PIR should be thought of as a camera of sorts. Most people have seen commercials on television where someone shows you a picture of a home and in that picture you can see where the heat is escaping. Of course they have a solution for that and for a small fee they can solve that problem for you. A PIR basically sees that same type of picture. It sees heat signatures. You may have a light bulb burning in the middle of a room, that is obviously a heat source and the motion detector will detect it but as long as it's not moving, there is no motion. When you walk across a room, your body temperature is detected by the PIR and that heat signature is moving so it trips the PIR.

For this reason it is very important that PIR's be positioned away from heating and air conditioning vents as well as return vents. Refrigerators, fire places, ice makers, and other appliances that may generate hot or cold air currents are also possible false alarm causes for PIR's.

Microwave motion detectors work exactly like a police radar. They emit a signal which is bounced off of objects back to the motion detector. A second signal is sent and if the time that it takes the second signal to bounce back is different than the time that it took the first signal, then something is moving. It's that simple. Microwave detectors have their place but they also have some draw backs. For one, microwave motion detectors can see though walls, so if you have one pointed at the front of the house, it may pick up a dog one the porch or a deer in the yard. Microwave motion detectors are also more expensive than PIR's.

Then there are duel tech motions. These motions use both microwave and PIR technologies. The big advantage here is almost no false alarms since both the microwave and the PIR technology must detect motion before the sensor will trip. Of course, these are more expensive still.

One last lesson on motion detectors. Since the PIR sees a "picture" and must see something moving in that picture, it is much easier to see something that is moving from side to side than to see movement from something that is moving straight toward the motion. For that reason, PIR's need to be mounted so that the likely intruder will be crossing it's path, not walking straight toward it. Unless the customer insists, we don't normally put a motion detector at the end of a hall since the intruder would be walking toward the motion detector instead of across it. On the other hand the microwave motion detector works just the opposite. It will pick up something more readily that is moving directly toward or away from it, than something that is crossing it's path.

Each technology has it's advantages and applications, give us a call and we'll match your needs with the components that fit best.