Passive Infrared Motion Detectors-
These detectors are also known as PIR detectors. The technology they utilize is "passive infrared". The device is mounted on a wall or in the corner of a room. It sends invisible fingers out into the covered area in several layers. The top layer goes the furthest and averages about 60 feet straight ahead and 35 feet on the sides.
The center layer of beams spreads the area about mid way and the bottom layer sweeps the room closest to the detector. These beams individually measure the infrared temperature of what ever they land on and look for a clash of temperature against that point. For example if a beam lands on your couch and knows what temperature it is, when you walk in front of the couch your temperature is different and causes a violation. You would be hard pressed to match the temperature of everything in your home as you walk about and that makes motion detectors hard to compromise.
Passive motion detectors have a microchip in them that will adjust the device for slow and methodical temperature changes. This way as your room warms up and cools down during an armed alarm period, you will not get a false alarm.
Some motion detectors are designed to be mounted in the ceiling and spread a 360-degree cone downward. Some are recessed to replicate an electric outlet and various other combinations are available for the James Bond like clients. Most often the device is an aesthetically pleasing small device that is mounted 5-7 feet high in the corner of a room.
Passive infrared motion detectors will not see through walls or windows as they will consider touching one of them as their final destination and begin calculating the temperature. The beams project forward only and will not bend around corners. If your device is placed where a beam can go into an area with an opened door, it will protect the interior of that area as well. Once the door is closed the beam will terminate on that door.
Motion detectors are not going to protect every square inch of your home or business unless you invest in many of them to accomplish that. Instead you should intelligently place them as an interior trap in an area or areas most likely to be violated by the creep or creeps that want to take your belongings or worst off violate you personally. Usually one placed properly on the main floor and one on the lower level if you have one, will serve as good traps. Stairways are often a good thing to consider when placing the device, due to the fact that you will prevent unauthorized passage from one floor to the next.
An alarm installer should always place a device where it will best serve the user and achieve the most coverage. Many an installer has elected to forgo this concept so that they can install the device in an area that is easier to get to with the wires. Insist on discussing all placement options with your installer before they place each motion detector. You can bet that your interests will be prioritized when they see your involvement in the decision process.
Dual Technology PIR-
Dual means two technologies are used in one device. Both technologies must be violated to cause an alarm. These devices are used in harsh environments such as a garage or sun- room. The first technology is passive infrared and works as explained above. The second technology is most often Doppler and looks for the invisible movement of air. If you walk into a room the air has to move as your body mass pushes it along. The reason you would want a dual technology device is clear when you apply common sense to the desired area of protection. For example let's say you pull your car in to a cool garage, go in the house and turn the alarm on for the night. Your motion detector that you put in your garage will see a dramatic temperature change as the heat from your engine radiates into the cool air. If you had a duel technology motion detector it would not see the air moving because your car is still, so it will refuse to go into an alarm condition.
Pet Immune Motion detector-
This is a wonderful advancement in motion detection that may work for you if your pet free- roams your house while you are away and have your motion detectors on. Before the introduction of this technology the pet owner had to either confine their pets from the protected area or bypass the motion detector rendering it useless unless the pet was out of the home with them. The technology is the same as the regular passive infrared detectors. On the pet immune version there are two sets of beams that are offset from each other. Your pet must hit two pre- assigned beams simultaneously in order to violate the detector. Pets under a certain amount of weight (up to 85 pounds) are not long enough to hit both beams so it does not see them. A human torso is designed much differently as per a weight to length ratio causing them to violate either a horizontal or vertical pair of beams, depending on their favorite burglarizing posture.
On the pet immune detectors the middle and lower span of beams are pet immune but the top layer is not, due to the distance of separation between the farthest-reaching beams. Care must be taken on the placement of these devices restricting the high beams from stairways and high ledges your cat may get up on. (6-7 feet high) A good technician will mask only the beams that hit these trouble spots expanding your coverage options.
Remember that even though your device is technically restricted for use by weight of your pet, two or more small animals will have an opportunity to hit the two proper beams while playing with each other. I do not recommend that you use these devices with two or more pets no matter how small they are, unless one of them is rarely moving about. Also one free flying bird will look like a dinosaur entered the room if it flies close to the detector. (They have yet to design the "Dinosaur Immune Detector")
If your pets do not fall into the allowances for using a pet immune motion detector then you should consider other options for creating interior traps. The well- designed system protects your perimeter as well as possible and creates interior traps in case the perimeter is circumvented.