The basic elements of the 'break-in' type of alarm are the sensors, the control unit, and the alarm bell, siren or buzzer. The alarm bell itself is fitted onto an outside wall - in a very prominent position so it acts as a deterrent to burglars. In areas of high crime, it's best to fit an alarm that is also set off if a burglar tries to remove the cover or cut the wiring circuit.
The control box is the 'brains of the unit: when a signal is received from one of the sensors, it sets off the audible alarm. Sensors can be magnetic window and door switches: movement of the magnet operates a switch, which sends an electrical signal to the control box, which in turn activates the alarm. The power supply can be either mains or battery - and some mains-operated systems have a battery-powered back-up, so that if power is cut off accidentally or deliberately, the battery takes over.
In most cases the alarm system is set with a key or a code number, which is input on a push button panel. If you live in a small flat or have a room in which precious items are located, there are also small, single-box devices available, which will set off an audible alarm by the movement of a door, window or by the sound of a break-in. There are also battery-operated door alarms with delay switches: these give legitimate occupants time to enter and leave a room without triggering it.
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