The reed switch is an electrical switch operated by an applied magnetic field. The switch is open when there is no magnet near to the switch and it will be close when magent is near to the switch.
NC (normal close), circuit is connected when magnet is near to the switch.
27mm x 14mm x 7.6mm
10 million times
1 Refer MAGNETIC DOOR SENSOR AND ARDUINO
2 Refer DIY Wifi Door Sensor