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These are the door contact sensors that are in use with any and all commercial or residential card access systems as well as all security burglar alarm system. The door position switches operate with a built an internal magnetic reed switch that closes when the opposing magnet comes within Â¾â of the door sensor. Hence when the door opens beyond the Â¾â point the door position switch will switch from a closed signal to an open signal. When this happens the access control system or alarm system will sense the change in continuity and will know that the door or window has been opened. When the door or window shuts, the door sensor will notify your security system that the opening has been closed.