Burglar alarms are sensor-driven systems that activate audible or visible alarms if they detect intruders or any attempt to remove protected items. The alarms may be located at the protected premises, or they may be at a remote monitoring station that supervises numerous alarm systems.
Mechanical sensors
The most commonly used sensor devices for the protection of doors and windows are magnetic reed switches encapsulated in the framework of a door or window and activated by a magnet fitted in the door or window itself. The switch remains closed as long as the magnet and reed are within ΒΌ in. (6 mm) of each other. If the magnetic field is removed by opening the protected door or window, the switch opens and activates an alarm.
A different type of sensor detects vibrations caused by intrusion or drilling. This type of alarm is commonly fitted to cars. False alarms owing to traffic noise or thunder can be minimized by carefully adjusting the sensitivity of the detector or by inserting a frequencensitive circuit between the sensors and the control.
Pressure mats, which detect the weight of an intruder, are generally constructed from two pieces of foil held apart by a sheet of perforated foam. The switch created by this arrangement is open until pressure forces the two pieces of foil into contact. A current then flows between the two sheets and the alarm is activated.
Movement sensors
A variety of ultrasonic and microwave movement sensors have now become standard for detecting the presence of intruders in protected rooms. The majority of these systems use the Doppler effect, whereby an ultrasonic or microwave signal transmitted by the system is altered in frequency when it is reflected off a moving object back to a detector. The change in the frequency of the return signal is detected and used to activate an alarm. In outdoor areas, this type of system can activate a floodlight that exposes intruders.
Ultrasonic and microwave movement sensors are prone to false alarms under certain conditions. Microwave signals, for example, penetrate glass and thin partitions, so movement outside the limits of a protected area may raise an alarm. Ultrasonics are contained within the protected area, but the system can be set off by high winds or by vibrations from, say, a doorbell ringing. In both types of systems, false alarms can be kept to a minimum by careful design and adjustment.
Closed-circuit television
Closed-circuit television (CCTV) systems are increasingly used in stores, office buildings, and public places. The views of several cameras can be shown in rotation on a screen, or an operator can select one particular camera when suspicious activity is identified. Video recordings can be used in evidence or in identifying an offender, either through public appeals for witnesses or through record searches.
In their early years, CCTV systems were useful only in well-illuminated areas. More recent systems use image intensifiers to produce images of reasonable quality using starlight and light reflected by clouds in the night sky. Another type of system uses infrared illumination and detection to produce images at night.
