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Overview
Wireless alarm systems usually contain a number of components, including sensors to indicate when a door opens or closes, signal arrays to convey radio alerts back to a central processing station, and laser-light "trip wires" designed to be stretched across a stairway or hall. The most important element, however, is probably a motion detector, which is able to monitor a wide area and capable of tipping alarms and floodlights when they are activated. Few comprehensive wireless alarm systems would be complete without at least one or two motion detectors covering central portions of the property.
Function
Most motion detectors work according to one of two principles: sonic signals and infrared detection. In the former case, the detector sends out signals that bounce off of inanimate objects in the area. As long as the pattern of those signals remains unaltered, everything is fine. When someone moves through the area, the signals bounce off the body and come back to the detector at a faster rate, altering the pattern and activating the alarm. An infrared detector works in a similar manner. It detects levels of infrared radiation, which are given out by the heat signatures of living things. When a being of sufficient size enters the area, the IR levels spike and the alarm goes off.
Considerations
In order to work effectively, motion detectors need to be placed in a location where they can do the most good. While inside, that usually means either a centralized room or a room that contains the most valuables (such as a bedroom or computer room). They work best when placed reasonably high, tilted at a slight downward angle, and situated near a corner where two walls converge. That allows most of the room to fall within its range without having to add another motion detector to augment it.
Features
Wireless motion detectors are battery operated, which means they operate on their own power source (typically either a common household battery or a specific battery included with the detector). Self-contained motion detectors will have their own alarm as well, though some may come attached with floodlights that will activate in the presence of an intruder. For motion detectors that are part of a large system, a signal device will be included, which can send radio signals to a central control panel that will then activate the alarm.
Benefits
Motion detectors have an advantage over door and window alarms simply because they cover more space. Monitoring a whole room or a back yard makes it much harder for a burglar to circumvent a single rigged entryway. Portable motion detectors bring further benefits because they can be detached from the wall very easily, allowing you to transfer the motion detector when you move without having to bother with wiring in the wall. Perhaps most important from a security standpoint, portable motion detectors are not connected to the house's power. So even if there's a blackout or a burglar cuts the electricity, the motion detector will continue to function.
Warning
The drawbacks to a wireless motion detector alarm are threefold. Most important, it cannot summon outside help when it is activated. While some wireless alarms can dial 911 and play a prerecorded message, most simply make a lot of loud noise. No security firm is monitoring it and if there's no one to hear the alarm, the burglar can continue on his way. Second, because it has its own power supply, the batteries must be rechecked regularly, and replaced if they are running low. Finally, a wireless system depends on radio waves, which means the motion detector must remain within a certain distance of the central control panel. If it passes beyond its range, it will not function.
