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 in internal magnetic reed switch that closes when the opposing magnet comes within ? of the door sensor. Hence when the door opens beyond the ? point the door position switch will change 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. · 3 Brown ABS Surface Mount Normally Closed Door Contact Switch · .2 Amp Maximum Switch Current · Will make Sense Door Closer at Roughly ? or 15mm · This Contact Works with Any Access Control or Burglar Alarm System Available · NORMALLY CLOSED: When the door or window is open, the contact will give an open signal and when the door is closed the contact will give a closed (circuit) signal.