Perimeter protection monitors the outside of a building for breaches, using an open-circuit or closed-circuit. A closed-circuit is the most common. A break in the circuit (i.e., a door or window being opened or broken) will trigger the system and set off an alarm. In an open-circuit system, a door or window that is opened or broken completes the circuit and trips the alarm.
Door and window sensors are a critical piece of the system. Sensors installed or embedded in a door for window frame align with a switch in the door/window. When the sensor and the switch are aligned, the circuit is closed. When the door/window is opened, the circuit is broken and this triggers the alarm.
Another type of door window sensor is a small button in the door/window frame that is pushed in when the door/window is closed. If the door/window is opened, the button is released and triggers the alarm.
Glass break detectors monitor sound levels and will trigger the system if a sound is above a certain frequency.
Motion sensors monitor activity on the inside of a building. These sensors work in a variety of ways: microwave energy sensors detect motion by looking for disruptions in the pattern of the waves the system puts out; photo sensors emit a beam of light and trigger an alarm when the beam is broken; passive infrared (PIR) sensors detect temperature changes within a space, like when a person enters an empty room.