Door and window sensors are comprised of two parts installed adjacent to each other. One part of the device is installed on the door or window and the other on the door frame or window sill. When a door or window is closed, the two parts of the sensor are joined together, creating a security circuit.
When the security system is armed at the control panel, these sensors communicate with it by reporting that the point of entry is secure. Should a monitored door or window suddenly be opened, the security circuit is broken and the control panel interprets this as a breach of a secured zone. A high-decibel alarm is sounded and in most instances the alarm monitoring company is automatically notified.