5 Sets MC-31B Magnetic Reed Switch Normally Open Closed NC and NO Door Alarm Window Security

A reed switch is an electrical switch operated by an applied magnetic field. Switch Type: NO/NC,  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 ). NO means Normally Open:when the magnet separate, the circuit is not conductive( light on );When the magnet together, the circuit is conductive ( light off ). Designed for use with alarm systems. Great for use in family and factory window, door. etc. Screw Mounted for easily install.