UHPPOTE NC Type Recessed Mounted Mortise Cylindrical Wooden Door Window Magnetic Contact Sensor Detector Reed Switch (Pack of 10) Wired wooden Door Sensor Recessed Mounted Materical: ABS housing This Contact Works with Any Access Control or Burglar Alarm System Available NC means Normally Closed: When the magnet together, the circuit is conductive (light on); When the magnet separate, the circuit is not conductive (light off).