A reed sensor is a device built using a reed switch with additional functionality or features, for example, to withstand higher shock, easier PCB mounting, additional intelligent circuitry, biased contact states, etc.
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The reed switch (sometimes also known as Herkon) is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferromagnetic metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay, or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.