Detecting Intrusions

We have already discussed the use of occupancy detectors to control lighting and heating, ventilating, and air-conditioning (HVAC) systems. They are also among the devices used to alert building owners and occupants to intruders.

Simple normally closed contact sensors include magnetic contacts for doors and windows, window foil, and pressure/tension devices. When the contact is broken, they transmit an alarm signal, thereby detecting an intrusion or damage to the system.

Mechanical motion detectors are used where window foil or fixed contacts are not practical. Mechanical motion detectors use a spring-mounted contact suspended inside a second contact surface. Motion on the surface on which the device is placed causes the contacts to come together momentarily and triggers an alarm. Mechanical motion detectors are very sensitive, and may be activated by sonic booms, wind, or a heavy truck passing the building.

Photoelectric motion detectors use a beam that opens a contact and initiates an alarm when it is interrupted. Modern photoelectric devices use lasers or infrared (IR) beams, which can be arranged to tell the difference between an intruder and other disturbances. Signals can be picked up, amplified, and retransmitted in a new direction, forming a perimeter security fence from a single source.

Passive infrared (PIR) presence detectors (Fig. 37-6) use the principle that all objects emit IR radiation (heat). Because objects change temperatures slowly, IR radiation changes very slowly in an undisturbed area.

High-sensitivity wide-angle p assive infrared detector

Passive infrared detector

High-sensitivity wide-angle p assive infrared detector

Passive infrared detector

Button-type ultrasonic detector for celling mount

Figure 37-6 Intrusion detection equipment.

Button-type ultrasonic detector for celling mount

Figure 37-6 Intrusion detection equipment.

Thus a rapid change in an IR reading indicates an object entering or leaving the space, and an alarm is triggered. PIRs are used as occupancy sensors to turn off lights when a room is unoccupied. They can also be used when motion in a monitored area is unavoidable. Rapid temperature changes caused by the sun, a cold breeze, or a heater turning on, however, can cause false alarms.

Some motion detectors operate at microwave or ultrasonic frequencies. When a moving object changes the frequency of a reflected signal, an alarm sounds. Motion detectors work best when located so that the path of an intruder is directly toward or away from the detector, not sideways to it. Ultrasonic detectors are cheaper than microwave detectors, but are disturbed by strong air turbulence and by very loud noises. Microwave detectors penetrate solids, and therefore may be affected by motion outside the protected area.

Acoustic detectors emit an alarm when the noise level exceeds a preset minimum. An acoustic detector may also respond to a particular range of frequencies, such as those of breaking glass or forced entry. They are also used as occupancy sensors for switching and lighting.

Using multiple detectors with differing techniques that can verify each other reduces the number of false alarms. Passive infrared and ultrasonic detectors are available installed in a single housing for this reason.

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