Tag Archives: Reed Switches

What are Reed Switches?

A modern factory will have several electronic devices working, and most of them will have several sensors. Typically, these sensors connect to the devices using wires. The wires provide the sensor with a supply voltage, a ground connection, and the signal output. The application of power allows the sensor to function properly, whether the sensor is sensing the presence of a ferromagnetic metal nearby, or it is sending out a beam of light as a part of the security system. On the other hand, simple mechanical switches, like reed switches, require only two wires to trigger the sensors. These switches need magnetic fields to activate.

The reed switch was born and patented at the Bell Telephone Laboratories. The basic reed switch looks like a small glass capsule that has two protruding wires. Inside the capsule, the wires connect to two ferromagnetic blades with only a few microns separating them. If a magnet happens to approach the switch, the two blades attract each other, making a connection for electricity to flow through them. This is the NO type of reed switch, and it is a normally open circuit until a magnet approaches it. There is another type of reed switch, the NC type, and it has one blade as a non-ferromagnetic type. This switch is a normally closed type, allowing electric current to flow until a magnet approaches it. The approaching magnet makes the blades pull apart, breaking the contact.

Manufacturers use a variety of metals to construct the contacts. This includes rhodium and tungsten. Some switches also use mercury, but the switch must remain in a proper orientation for switching. The glass envelope typically has an inert atmosphere inside—commonly nitrogen—to seal the contacts at one atmospheric pressure. Sealing with an inert atmosphere ensures the contacts remain isolated,  prevents corrosion, and quenches sparks that might result from current interruption due to contact movement.

Although there are solid state Hall effect sensors for detecting magnetic fields, the reed switch has its own advantages that are necessary for some applications. One of them is the superior electrical isolation that reed switches offer compared to what Hall effect sensors do. Moreover, the electrical resistance introduced is much lower for reed switches. Furthermore, reed switches are comfortable working with a range of voltages, variable loads, and frequencies, as they function simply as a switch to connect or disconnect two wires. On the other hand, Hall switches require supporting circuitry to function, which reed switches do not.

For a mechanical switch, reed switches have incredibly high reliability—they typically function for billions of cycles before failing. Moreover, because of their sealed construction, reed switches can function even in explosive environments, where a single spark could generate disastrous results. Although reed switches are older technology, they are far from obsolete. Reed switches are now available in surface mount technology for mounting on boards with automated pick-and-place machinery.

The functioning of reed switches does not require a permanent magnet to actuate them. Even electromagnets can turn them on. Initially, Bell labs used these switches abundantly in their telephone systems, until they changed over to digital electronics.

Using Reed Switches as Sensors

Any ordinary electrical switch has two contacts. Push-type switches are spring loaded so that pushing a button brings them together and they spring apart on releasing the button. Rocker switches have mechanical levers that close the contacts when in one position, while in the other position they pull apart.

In reed switches, the two contacts are in the shape of metal reeds, each coated with a metal that does not wear easily. The reeds are made from a ferromagnetic material, so they are easy to magnetize. The entire assembly is hermetically sealed within a thin glass envelope containing a nonreactive gas such as nitrogen. For extra protection, sometimes the glass envelope may have a plastic casing.

The ferromagnetic material making up the reeds is typically a nickel-iron alloy that shows high magnetic permeability but low magnetic retentivity. That means, when brought close to a magnet, it magnetizes the reeds, which come together in contact. On moving the switch away from the magnetic field, the reeds lose their magnetic property and separate. Their movement has high hysteresis, that is to say they close and open slowly and smoothly. The reeds have a flat area where they contact each other, and this helps to extend the life and reliability of the switch.

Although reed switches typically have two ferromagnetic contacts, some variants may have only one ferromagnetic contact, while the other is non-magnetic. Others may have three contacts, with two non-magnetic and the central one as ferromagnetic.

Like ordinary switches, reed switches also come as two major variants—normally open type and normally closed type. This refers to the position of the reeds when there is no magnetic influence on them. Therefore, the normally open type has its reeds separated from each other, and they close when a magnet is brought close enough. The normally closed type of reed switch has its reeds in contact with each other, and they move apart when a magnet is brought close enough.

As the magnet comes close to a normally open reed switch, the two contacts become magnetized as opposite magnetic poles, and they attract each other to close. In this position, the switch can pass an electric current. This magnetizing of the reeds is independent of the pole of the magnet coming close to them. As the magnet moves away, the reeds lose their magnetism, and their stiff and springy nature makes them spring apart in their original position.

Reed switches are very useful as sensors such as for sensing level of liquids. A sealed stem holds the reed switches at different heights. A float containing a permanent magnet rides on the stem, going up and down as the liquid level changes. When the float magnet comes close to one of the reed switches, it snaps close, changing its electrical status that any electronic circuit can sense. Automotive, marine, and industrial applications use reed switches for level sensing.

A float switch in a dishwasher controls the level of water in the machine. The shaft containing the reed switch is positioned at the water fill limit of the pan. As the water rises, so does a float containing the magnet. When the magnet comes close to the reed switch, it closes, and signals the ECU.

Advanced Applications Need Alternate Switch Technologies

Although conventional reed switches have been in use for their excellent properties, their large size makes them difficult to integrate in advanced applications. Most equipment now use miniature components and manufacturers have found a way to reduce the size of reed switches to match. They now use HARM MEMS or High Aspect Ratio Microfabrication MEMS technology to make miniature reed switches, keeping all their desirable properties intact.

Reed switches are popular because they do not require power to operate, they offer milliohms of ON resistance, and tera-ohms of insulation when OFF. They are immune to ESD or electrostatic discharge. Moreover, they require very little additional circuitry to operate and hence, take up very little real estate on the printed circuit board. Some advanced applications where the alternate HARM MEMS reed switches are useful are as follows.

Small Portable Hearing Aids

The baby-boomer market is increasingly in need of small portable medical devices such as hearing aids or hearing assistance devices. HARM MEMS switches are ideal for the control functions in these devices. As the user preference for small, almost unnoticeable hearing aids grows, the ever-shrinking devices are making increasing use of the tiny magnetically operated switches for functions such as Telecoil operation and program switching. As these switches need no power to operate, the once bulky behind-the-ear hearing aids are disappearing into the ear canal itself. Since batteries have also shrunk, the zero power operation of the microfabrication reed switches is a boon for the user.

Endoscopes the Size of Capsules

No one forgets the trauma of getting an endoscope done to know what is wrong within his or her gastrointestinal tract. However, that might soon be outdated, as HARM MEMS can shrink the endoscope down to the size of a capsule, which the patient swallows. As the pill shaped endoscope passes down the gastrointestinal tract, its one or more video cameras capture images lit by its white LED headlamps, also a part of the pill.

As the device is small enough to be swallowed easily, the capsule endoscope has electronic circuitry that is highly miniaturized, so that it can reach where conventional endoscopes or colonoscopes cannot. The tiny pill requires a mechanism to allow it to start functioning just before it is swallowed. In addition, there must be no drain from the tiny batteries when the device is in storage. Active switches are not helpful here, as they draw current even when inactive and hence reduce the shelf life. HARM MEMS switches are the best fit here because of their tiny size and zero power consumption.

Insulin Delivery Pumps

All over the world, diabetes is increasingly affecting people of all ages. In the most severe form of this disease, insulin must be administered multiple times daily to the body. There are two ways to do this – either by multiple daily syringe injections or via insulin pumps. The pumps generally contain a disposable insulin reservoir. The pump unit must reliably detect this reservoir. Modern insulin pumps are small credit card sized and contain a HARM MEMS reed switch, which is activated by a magnet attached to the reservoir.

Integrating Piezoelectric Flexure Actuators

The familiar reed switch comes with a unique set of properties. These include ON resistance of the order of milliohms, OFF resistance of the order of tera-ohms, total immunity to ESD or electrostatic discharge, hot switching capability of about one watt, and almost zero power operation. However, as all electronic components are shrinking to surface mount sizes of 0402, 0201 and even to 01005 (0.4 x 0.2 mm), the large size of the reed switch is anachronistic. Since 70 years of its invention, the conventional reed switch has been steadily shrinking. What began with a 50 mm long glass tube in 1938 has come down to about 5 mm today.

However, even after a sort of following Moore’s law of ever-shrinking transistor size on integrated circuits, reed switches have now reached a brick wall. The fundamental limitations of physics and manufacturing are preventing the conventional reed switches from going below the 5 mm size. Now, a new technology promises to break this barrier of 5 mm size, while retaining all the desirable properties of the reed switch. Manufacturers are using HARM, or High-Aspect Ratio Microfabrication MEMS technology. For instance, reed switches such as the RedRock RS-A-2515 piezoelectric flexure actuators from Coto Technology is based on this technology.

Alternatives to reed switches also exist. For instance, there are Hall Effect switches, AMR or Anisotropic MagnetoResistive switches, and GMR or Giant MagnetoResistive switches in the market. However, all the above are active switches, requiring a power supply to operate them. This is a disadvantage related to these active switches, as they add to circuit complexity and take up PCB real estate. Active switches require three electrical connections instead of two – one for supply power, one for the return ground and the third for the sensor signal.

Active switches have further disadvantages in that they require external circuit elements such as bypass capacitors or pull-up resistors. This increases the cost and effective size of these multi-component switching systems. There is additional complexity as these can only switch milliamp-level currents, and extra buffer circuitry is necessary for switching higher currents. Active switches are also susceptible to damage from ESD. In contrast, reed switches made from the HARM MEMS technology has none of the above disadvantages.

Switches made from the HARM MEMS technology offer very small size, high-current hot-switching capability, and zero-power operation. This performance makes the technology suitable for a wide range of applications including automotive and medical. For instance, HARM MEMS technology allows making endoscopes the size of a pill that patients simply swallow, nearly invisible and tunable hearing aids, convenient insulin delivery systems, and some exciting new automotive switching applications.

Although motor vehicles are large systems with enough battery power, conventional affordable reed switches have been widely used for a variety of functions such as ABS systems, gear lever position sensing, and door lock control. Smaller reed switches are also necessary in vehicles for sensing various fluids, for instance, brake fluids. Usually, a single reed switch, triggered by a float magnet in the fluid reservoir indicates a binary position – there is either enough fluid, or there is none.