Magnetic Detectors in a closed circuit consist of a few basic components. Typically they consist of:

• a powered circuit
• a spring-driven metal switch in the door frame
• a magnet in the door, aligned with the switch
• a separately-powered buzzer with a relay switch.
  

When the door is closed, the magnet pulls the metal switch into the closed position to complete the circuit. The current then powers the relay's electromagnet, so the buzzer circuit remains open. When you move the magnet by opening the door, the spring snaps the switch back into the open position. This cuts off the current and closes the relay, sounding the alarm. You can also build this sort of system into a window. If an intruder pushes a window open, the magnet slides out of line with the switch, and the buzzer is activated.