Figure 8-21. - Recessed magnetic contacts in casement window.

Other space detectors include ultrasonic motion detectors, audio detectors. and infrared detectors. Care must be used with any of these units because the protected area is limited both in width and depth-depending upon the particular unit.

The ultrasonic motion detector can be used in large glass-walled rooms that might otherwise be difficult to protect and in hallways or entries or in virtually any area an intruder would have to pass through in moving about a home or business. They are especially useful as added protection (when conventional detectors are used also) to monitor a "valuables" room or area.

Most ultrasonic motion detectors are designed for mounting on either the wall or ceiling. It emits inaudible high-frequency sound waves in an elliptical pattern that ranges from 12 feet (4 m) to 5 feet (11 m) long, by 20 feet (6 m) wide, by 5 feet (2 m) high for most residential models. When an intruder moves within the secured area, movement interrupts the established pattern of sound waves and sounds the alarm.

Some designs of motion detectors can be rotated up to 180° for maximum coverage of the area being monitored. as shown in figure 8-28.

Another type of motion detector is the audio detector (fig. 8-29). This type senses certain sharp sounds known to be present in forced entry, such as wood splintering or glass breaking. When these sounds are received through the miniature microphone of the unit, the detector triggers the control unit to sound an alarm.

Audio detectors are best utilized in areas that are seldom used, such as an attic, a garage, or a closed-off wing. They can be used in other areas, but when such areas are subject to much daytime activity, it is recommended that the detector only be armed at night when the business is closed or the family retires or is away from home.

Infrared detectors are another type of motion detector. A combination transmitter-receiver is used to project an invisible pulsating beam at a special bounce- back reflector on an opposite wall. Any interruption of the beam activates the system alarms. Infrared detectors can be wired to either the perimeter or interior circuit; but for faster response, it is recommended that it be connected to the interior circuit.

Infrared detectors are designed for indoor areas, such as entries, hallways, rooms, and so forth. Most cover a span from 3 feet (1 m) to 75 feet (23 m), so it may be used in practically any indoor area or room.

Figure 8-22. - Conductive foil on glass doors.