Answer:
A network of PIR's (Passive Infra Red) detectors, panic attack buttons, door magnets, sirens/strobes and a central control unit/panel powers the system. The user will type in a personalized code into the panel keypad, giving the user a certain amount of time toexit the building before the alarm system is set. Then, when a person enters through a door, the magnetic door contacts detect the door movement. This happens when the magnets move apart from eachother. As when the door is closed, they are both next to each other. When the door is opened, it sends a signal to the control unit giving the user a certain amount of time to type in the code. If the type the code in right, then the system becomes unset and it is safe to walk around the house. But if the period of time is up and you have not typed in the code or you typed the code in wrong, then the burglar alarm will sound. The PIR's detect physical movement in rooms, corrridors and hallways etc. The same principle applies with the door contacts. When they detect movement when the alarm is set, you have a certain amount of time to unset the alarm. Different times vary with different alarm systems.