Sensors & Switches

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Elwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. more...

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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 reed relay is a type of relay, in which a reed switch is mounted inside a coil..

Description

The reed switch contains two magnetizable and electrically conductive metal reeds which have end portions separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope.

A magnetic field (from an electromagnet or a permanent magnet) will cause the contacts to pull together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. A more complicated configuration contains a non-ferrous normally closed contact that opens when the ferrous normally open contact closes. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds. There are also versions of reed switches with mercury "wetted" contacts.

Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard.

One important quality of the switch is its sensitivity, the amount of magnetic energy necessary to actuate it. Sensitivity is measured in units of Ampere-turns, corresponding to the current in a coil multiplied by the number of turns. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range.

In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed. Infrared-absorbing glass is used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components such as lead oxide and fluorides. The leads of the switch must be handled carefully to prevent breaking the glass envelope.

Reed switches are widely used for electrical circuit control, particularly in the communications field. Reed switches are commonly used in mechanical systems as proximity switches as well as in door and window sensors in burglar alarm systems and tamperproofing methods; however they can be disabled if they are in a strong, external magnetic field. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key. Speed sensors on bicycles use a reed switch to detect when the magnet on the wheel passes the sensor.

Read more at Wikipedia.org