The burglar alarm control panel is the most complex part of the standard alarm system, and usually the most expensive single component. In effect it is a computer which links together all the various detection devices in the system, at the same time being the interface between the user and the system. All of the sensors, such as magnetic contacts, PIR detector, pressure mat and window foil are wired back to the control panel, as is the alarm bell or siren. There are a plethora of sensing devices available for modern alarm systems:
MAGNETIC SENSORS
Many alarm systems contain a magnetic sensor. This sensor is utilised to detect the opening of a window or door. In addition, there is usually a magnet positioned permanently on the door or window frame. There is also a special switch situated with the magnet. If the door or window is opened after the alarm has been set, the magnet shifts and the metal bar will no longer be held. A spring draws the metal bar backwards and causes the contacts to open. The opening of these contacts cuts the electrical circuit and triggers the alarm.
BREAK GLASS DETECTOR
The unit is mounted on a wall or ceiling and will then monitor an area, listening for the high and low frequency sounds made when glass is broken. When a window is broken, the sounds emitted are high frequency. These are followed by low frequency sounds as the broken glass drops onto the floor. The sounds must be of the correct frequency, at the correct level and in the correct order before the unit will trigger an alarm. It is this accuracy which prevents the false alarms associated with the more primitive versions.
WINDOW FOIL DETECTOR
With the PIR (passive infra red) sensor and break glass sensor becoming more reliable and more affordable, some methods of intruder detection are moving towards obsolesence. The window foil is one of these. It consists of a strip of aluminium foil, usually with a self adhesive backing and two terminations, one at each end. The strip is stuck to a window and from each terminal a wire is run back to the control panel to form a continuous loop, or closed circuit. The aluminium acts as a conductor of electricity across the window. Then if the glass is broken, the theory is that the foil would tear, breaking the circuit and setting off the alarm.
PIR DETECTORS
As infra-red beams are invisible to the human eye, they are often used within sensors for alarm systems. The typical infra-red sensor consists of a transmitter which creates the beam, either straight from a gallium arsenide light source or by filtering a bright light source. A receiver is visually lined up with the transmitter. Depending on whether the receiver continuously receives the infra-red signal, a photo diode detector will generate a signal to hold the device in a non-alarm state. If this beam is interrupted at any time the alarm will then sound. PIR (Passive Infra Red) detectors are the most common form of security system sensor in use today, giving a wide coverage of open space from a single point via the omni directional infra red beam.
