This Reed switch module used a small device called a reed switch. When the device is exposed to a magnetic field, the two ferrous materials inside the switch pull together and the switch closes. When the magnetic field is removed, the reeds separate and the switch opens. This makes for a great non-contact switch. It can apply to PBX, photocopiers, washing machines, refrigerators, cameras, disinfection cabinets, doors, window magnetics, electromagnetic relays, electronic weighing, level meters, gas meters, water meters, etc.