Contacts
Various types of door / window contacts are available. They are available in white or brown plastic and surface or flush mount. Surface mount is needed for a UPVC double glazed door because there is nowhere on them to drill and sink anything into. Flush contacts can be used on a wooden door and frame so that the device is sunk into the wood and therefore not visible when the door is closed. When the door is closed the two parts of the magnet line up with each other and create a circuit, the alarm operates if the door opens because the circuit is broken. If the protected door is the main entrance door than we can programme the system to allow a delay of enough time to allow the user to unset their alarm. If the alarm has not been unset before that time expires the alarm will activate. In addition roller-shutter contacts are available for garage doors (see picture) and read more about them in the grade-3 detectors section