Burglar alarms are triggered by either opening or closing a circuit. Systems can be either closed circuit or open circuit, but closed circuit systems are the more popular and secure choice.
In a closed circuit system, the electricity can flow between the two circuit ends when the door is closed. Opening the door causes a break in the flow and triggers the alarm. In an open circuit system, the alarm is triggered when the door is opened and the circuit is completed.
Circuits consist of a battery, magnet, spring activated metal switch and a separately powered buzzer. The magnet pulls the switch closed so the circuit is complete when the door is closed. When the door is opened, the magnet moves and the spring snaps the switch open, triggering the alarm. This same system can also be applied to windows.
The burglar alarm control box is hooked up to and monitors all alarm circuits on a property. When the alarm is triggered, it only shuts off when a unique security code is entered into the keypad.
A simpler alarm system uses a button switch. The button is pressed in when a door is closed and is released when the door is opened and triggers the alarm. However, due to the fact that the alarm can be shut off by closing the door again, this system is much less secure.