False Alarms

A burglar alarm can initiate a considerable response by police or private security personnel, who may leave other important duties to race to the scene of the alarm. Therefore, it is important to prevent false alarms when designing and installing alarm systems. Modern burglar alarms make use of several different technologies to reduce false alarms.

Passive infrared detectors can be programmed to ignore the first movement detected, as in when the intruder moves from one detection zone to another, and to sound the alarm only when the movement passes through two or more detection zones within a specified period of time. In this way, an insect landing on the detector’s lens, or a sudden rise in background temperature caused by an activated furnace, is ignored.

Passive infrared detectors can be programmed to ignore the first movement detected, as in when the intruder moves from one detection zone to another, and to sound the alarm only when the movement passes through two or more detection zones within a specified period of time. In this way, an insect landing on the detector’s lens, or a sudden rise in background temperature caused by an activated furnace, is ignored.

Dual Technology

Another means of preventing false alarms is the dual-technology motion detector. This is probably the most common type of detector used in more sophisticated burglar alarm systems. A dual-technology detector combines a passive infrared device and a microwave device in one small unit. The passive infrared device sees many detection zones and measures the change in background temperature as a target moves across them. At the same time, the detector projects microwaves and measures the Doppler shift when a target moves through the protected space.

An infrared motion detector will detect movement regardless of whether the target is moving across the field of view or toward the detector. But such a detector is more sensitive to movement across its field of view. Thus, it is more prone to false alarms caused by disturbances such as a mouse or rat moving across its field of view than by movement toward it. Microwave detectors are just the opposite: more sensitive to targets moving toward them than they are to targets moving across their field of view. If a large leaf falls off a plant in a room, a microwave detector is more likely to detect the motion than is an infrared detector. But if there is movement outside a window, a microwave detector might detect it when an infrared detector probably would not.

Dual-technology motion detectors use a circuit that requires both devices to detect motion before an alarm is sounded. A bird landing on an outside windowsill might trip the microwave device but not the infrared device, so no false alarm would be transmitted.