uxcell RC-33 NC Recessed Wired Security Window Door Contact Sensor Alarm Magnetic Reed Switch Brown ABS Normally Closed Door Contact Switch 0.5A Maximum Switch Current Will make Sense Door Closer at Roughly 10mm 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 ).