sensor. Metal surface should be avoided. Installation should always be
specified on the inside surface (within the protected area).
b) Grid-wire sensors. Grid-wire sensors are a wire mesh
embedded in or affixed to building barriers which cause an alarm to occur
when broken. These sensors must be embedded in the barrier to conceal the
a) Balanced magnetic switches. Also referred to as door
"switches," this sensor is the most commonly used intrusion detection device.
The standard magnetic switch sensor consists of two components: one contains
contacts that open or close in the presence of a magnetic field, the other
contains the magnet which provides the magnetic field. It is possible to
defeat a "plain" magnetic switch by placing a strong magnet near the switch,
thus preventing the contacts from activating when the normal switch magnet is
moved away, as when the door opens. As a result, the only acceptable type of
magnetic switch for most DoD applications is the balanced or biased magnetic
breakwire sensor. This type of sensor is used to
detect breakage of glass by
an intruder. The breakage of glass also causes
the breakage of a thin, low
tensile strength wire embedded in the window
mullions or overlaid on the
glass itself. This breakage interrupts a low
voltage direct current that
runs through the wire, generating an alarm.
c) Window vibration/ultrasonic sensors. Both of these
sensors are known as "window bugs." Vibration sensors detect attempts to
penetrate windows. Ultrasonic sensors detect the sounds made by forcible
intrusions even if the sounds are not audible to the human ear. Such sensors
are passive receivers consisting of a microphone and an electronic processor
which discriminates between "noise" such as human speech, and the specific
frequency associated with forcible entry attempts.
Detection Within Building Interior Volumes. Volumetric detectors
detect the presence of an intruder within a given volume of space. The
following are the most commonly deployed.
Ultrasonic. This sensor emits ultrasonic energy (inaudible to
the human ear) and sets up a "standing" energy field which is sensed by a
receiver. This sensor works on the Doppler principle. That is, any movement
in the field produces a frequency change which is detected by the receiver
and causes an alarm. Electronics within the sensor permit adjustment of the