Overview of the Reed Switch

A Reed Switch contains two very small ferromagnetic paddles or “reeds” inserted into either end of a tiny glass capillary. The glass casing is hermetically sealed during manufacture, with switches varying in length from one fortieth of an inch to two inches.

Placed inside a magnetic field, the ferromagnetic paddles will be induced to move together until the contacts at either “free” end in the mid-region of the capillary close, completing a previously broken circuit and enabling a switching function. Once the magnetic field is removed, the rigid paddles separate again, so that the circuit is again broken.

There are two main types of Reed Switch – one which contains two ferromagnetic paddles which in the absence of a magnetic field are normally open (“NO”) as just described, and another which contains three. The latter has three paddles: a single reed enters the capillary at one end and two enter from the other, creating two contact surfaces in the middle. In the absence of a magnetic field the single reed touches the “normally closed” (contact) but switches position to the “Normally Open” contact once the field is present. The reverse happens when the field is removed.

A microscopic layer of precious metal is “sputtered” over the reeds’ contacts in order to guarantee optimum electrical contact, with low-resistivity silver being the favoured metal. Some Reed Switches, however, utilize mercury. Because the contacts are “wetted” with the mercury, these switches must be held in specific orientations when mounted to prevent the liquid metal from dripping and bridging the contacts when not in use.

               

History

The idea for the reed switch crystallised in the mind of Professor V. Kovalenkov in Russia in 1922 during his tenure at Leningrad Electrotechnical University, where he lectured on magnetic fields. But in 1936, the US company Bell Telephone Laboratories began investigating and testing reed switches. Four years later in 1940, reed switches began to be manufactured there.

By the 1950s, reed switches were being used in the construction of quasi-electronic exchanges with a speech channel (today, we would simply call them telephone exchanges), and in 1963 Bell built the world’s first intercity quasi-electronic telephone exchange. A speech channel of such magnitude required no less than 690,000 reed switches.1000 such exchanges were in use in the USA alone in 1977. By that year, demand for reed switches had burgeoned – the Hamlin Company alone manufactured 25 million of them.

They are widely used today in myriad communication devices, and as proximity sensors on door and window burglar alarms, to mention but two of many different applications.

 

Technical aspects

Because international demand for reed switches is so vast they have to be mass-produced, but they require sophisticated micro-technology to make them accurately and reliably. A super-clean environment free from even microscopic particles of contamination is necessary, otherwise contaminants would become trapped in the hermetically sealed glass capillary and impairs the functioning of the switch.

The leads of the ferromagnetic reeds are manufactured from an alloy of Nickel and Iron, 52 per cent of which is Nickel. The contacts are “sputtered” with a microscopically thin layer of iridium, rhodium or ruthenium, a coating which is applied on top of an under-layer made from Tungsten, copper or gold.

The glass capillary, whose Temperature Coefficient of Expansion is identical to that of the Nickel-Iron alloy, is sealed around the reeds at each end by heating them with laser-focused infrared radiation until the aperture closes. While sealing is underway, the glass cavity is filled with an inert gas such as nitrogen.

The point at which the contacts in a reed switch close is known as the “Pull In” or PI, while the point of opening is called the “Drop Out” or DO. The switch’s sensitivity refers to the strength of magnetic field needed to induce PI, and most reed switches have PI sensitivities ranging between 10 and 60 Ampere Turns or AT (this is essentially derived by multiplying the current in the coil by the number of turns). Smaller switches are usually considerably more sensitive to magnetic fields than larger switches.

 

Where the micro switch is used in manufacturing

Still widely used to control electrical circuits in the communications industry, Reed Switches are also ubiquitously deployed in burglar alarm systems as proximity sensors, typically being positioned on windows and doors.

They can also be found activating the hibernation mode on laptop computers whenever the lid is closed, and were widely deployed in computer keyboards – each pad had an integral magnet which would activate the switch as soon as a key was depressed. The latter have largely given way to less expensive alternatives today, however.

In cars and bicycles, magnets fitted to the wheels activate reed switches every time they pass the switch’s sensor, enabling them to function as accurate speed sensors.

In addition, equipment used in high-pressure water environments such as diving cameras and flashlights are typically fitted with reed switches to help maintain a watertight seal.

 

How the reed switch differs from other switches

By contrast with other electrical switches, the reed switch is specifically designed to respond sensitively to the presence and absence of a magnetic field, a characteristic which has led to its utilisation in myriad applications.