A reed switch is a sensor that closes circuit in the presence of a magnetic field. Reed sensors can be used in many application where contact less On/Off is required. However reed switches can be fragile to use directly, hence this module can make it easier to handle and mount the sensor in various applications.
 
It is a kind of passive electronic switching component with contacts with a simple structure, small size and easy to control. It consists of a sealed glass envelope where there are two ferrous elastic reeds and is filled with inert gas called rhodium.
 
It gets operated by the applied magnetic field. 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.