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
Figure 8-21.Recessed magnetic contacts in casement
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)
may be used in practically any indoor area or room.
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
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
Figure 8-22.Conductive foil on glass doors.