There is elegance in simplicity, and few things are as elegantly simple as a reed switch. The technology is basic and time-tested, and has not changed much in the seven decades it’s been around. But, because of their versatility, product designers are continually finding new uses for the tiny devices. And that’s one reason why these millimeter-sized devices are sold by the hundreds of millions each year. Two other important reasons: reed switches have extremely long life compared to other electromechanical switching technologies and they don’t corrode, despite often being asked to go into harsh environments.

A reed switch is a basic switching device that is actuated by a magnet. The switch is made from two slim, ferromagnetic blades — the reeds. The reed ends are plated with a thin layer of precious metals that ensure a good electrical contact. The switches are encapsulated in a hermetically sealed glass enclosure with an inert gas, typically nitrogen, which will minimize the possibility of electrical arcing.

A benefit of reed relays are their tiny size. Photo by Meder.

In its simplest, two-reed form, actuation occurs when a magnet passes near the reed and the two blades are drawn to each other to make electrical contact. When the magnet passes the reeds, the reeds spring back to their original position and the contact is broken. Additional versions are available depending on the requirements of the application. Two-reed versions, which typically have normally open (NO) contacts, are also available with normally closed (NC) contacts. In this case, the reeds are held together by a permanent magnet in their normal state, and another magnet with opposite polarity is needed to actuate it. There are even three-reed versions that have a pair of normally open and normally closed contacts.

Reed switches can be as small as a couple millimeters long and they typically can handle only a couple of amps, but this, too, may be changing. Reed Switch Developments Corp., Racine, Wis., a company that develops reed switch applications employing multiple vendors is using what it calls the one of the largest capacity reed switches in the world on projects for two of its customers. The reed switch, made by Japan-based Yaskawa, operates at 240 VAC, 10 A Maximum.

The leads on this Hamlin Flex reed switch can be bent to fit.

While reed switches do not have the sophistication of solid-state switches and relays such as a Hall Effect relay, the simplicity of a reed switch remains its key strength, says Simon Pitkin, the UK-based sales and marketing manager for Hamlin Electronics. “Cost is really quite a significant issue,” he says. “Reed switches can be less expensive than competing technologies and there is no current drain as a result of using.”

Because of its relative simplicity, a reed switch has a life cycle in the tens of millions of operations, though long cycle life is not necessarily the most telling quality of a reed switch, says Tab Hauser, president of Hasco Components International, New Hyde Park, N.Y.  He says that what makes a reed switch unique is its reliability when needed.

The switches are environmentally insensitive, unlike a pushbutton or toggle switch. The plated-tipped reeds eliminate the worries about contact resistance or corrosion. “Reed switches are a totally non-corroding switch,” he says. “Anybody can make a reed switch that can go more than 1 million, or even 10 million operations, but a good reed switch is one that can sit idle for five years and the one time that it is needed it will work without sticking.”

Reed switches come with straight leads out of the box, but they can be cut and bent to fit in PC board holes or made to go surface mount. In fact, one company, Hamlin Electronics has launched a new product that allows the customers to bend the switch lead themselves without damaging the switches’ glass-to-metal seal. In the past, when trying to bend the leads the glass-to-metal seal might get broken, so suppliers often bent them to specification before shipping.

The sensitivity of a reed switch, that is, the amount of force required to draw the contacts together, is measured in Ampere turns (AT), which correspond to the current in a coil multiplied by the number of turns. The switches often come in a range of 10 AT to 60 AT in increments of 5 AT. In the case of Hasco, the increments are 2.5 AT.

In general, the lower the AT number, the more sensitive the switch, says Hauser. Sensitive reeds are often preferred because they do not require as powerful of a magnet. Hauser adds that there are applications where less sensitive magnets might be preferred such as multiple switches that are situated in rows along a track. “A dishwasher or laundry machine may use them in a track to monitor the fluid level, and you don’t want the magnet to click off two reed switches at the same time.”

One of the benefits of a reed switch is that it can be used in underwater environments as they can withstand up to 5,000 psi, says Rick Berns, vice president and sales engineer for Reed Switch Developments. Because the reed switches are hermetically sealed in a glass environment, they remain free from contamination, and are safe to use in harsh environments. In an appliance application, such as a dishwasher, the magnet would be placed on a float and as it passes the reed switch, it actuates the circuit that stops the water-fill part of the cycle.

Novel uses

Reed Switch Developments uses a variety of switches from multiple vendors for novel uses.

When it comes to reed switches, home appliances are not normally the first applications considered. Reed switches, which were first invented in 1936 in a Bell Telephone Laboratory, are still used in the telecommunication industry. They are also widely used in security applications to detect the opening of a door or window. Many home security systems will have a reed switch in a doorframe and a magnet embedded in the door edge.

While those are the more commonly known uses, applications for reed switches are becoming more widespread. Hasco sells hundreds of thousands of reed switches to the garage door opener market — a contract Hauser won after complaining to the manufacturer about his problematic door opener. The switch had been sticking and not shutting off the motor.

Hamlin Electronics supplies reed switches for air conditioners, vacuum cleaners and other household appliances; products in the medical field such as electrically operated hospital beds; and even in  exercise bicycles found at local health clubs.

Reed Switch Development found some of its switches in a riding lawn mower application that sensed if a rider were in place atop the mower. If the operator were to fall, the reed switch would trip and stop the blades. In another application, the company sold switches to elevator manufacturing company. The switches were installed to ensure that the elevator was level with the floor before the door opens.

Rob Greenberg, marketing/national distribution manager, for Meder Electronic, Mashpee, Mass., points to a number of novel ways to use a reed switch. For example, an electric toothbrush is subjected to acidic human saliva, toothpastes, which are generally alkaline, and are constantly subject to hot and cold water. The most vulnerable point is the on/off switch, but Greenberg says this vulnerability can be addressed with a reed switch and magnet combination. The magnet is molded into a plastic slide, which moves up and down. Inside to the electric toothbrush, and mounted on a pc board, is a reed switch that senses the position of the magnet. In the down position, the switch remains open; in the up position, the magnet activates the reed switch or triggers the motor. The housing remains sealed, not needing an opening for a switch mechanism.

Reed switches such as these from Hasco come in a variety of bent configurations.

In another Meder application, reed sensors are used to control the various power settings for stovetop burners. The sensor eliminates the need for knobs, electronic gadgetry and Hall effect sensors; the latter two of which require added power and additional circuitry. On the surface of the oven and near one of the elements, a sliding device with an integrated magnet is installed. Reed switches are mounted just below the surface. As the sliding device is moved, it triggers the various reed switches, which in turn will activate different power settings.

One of the most common applications for a reed switch is in the form of a reed relay, which is made from a reed switch with a coil wrapped around it. When a current is sent through the coil, it becomes magnetized and the reed switch closes. According to Hauser, the reed switch is usually molded in a single in line (SIP) or dual in line package (DIP), and is very fast. Hauser, whose company offers reed relays in versions including DIP, SIP, RRH and mini-reeds, says the relays can operate at speeds of 0.3 milliseconds, and can handle switching current from 0.3A to  1A and carrying current from 0.5A to 2.5A. Reed relays are also available for PC boards. (A carry current is the maximum current that can be applied to an already closed contact. The switch current is the maximum current a reed contact can switch.)

Meder Electronic recently released its RM series of reed relays that features a low-profile module having three input-output options — four inputs with four, two and 1 outputs — and is capable of switching beyond 3 GHz with <40 ps rise time.

Another company with a new reed relay product is Warwick, R.I.-based Coto Technology that offers reed relays that are capable of carrying up to 6 amps. The relays can provide switching current up to 4 amps along with 10-15 KV switching. One of the company’s newest products is the Coto RI-90 Series with a 15 mm maximum glass length, which is a single-pole double-throw type, having three contacts: a common contact, a normally-open contact and a normally-closed contact. When brought in the proximity of a magnetic field, the common contact will first open with normally closed contact, and then make contact with the normally open contact as long as the magnetic field is applied.

Trial and error

A strawberry drawfs this reed switch from Meder.

Selecting the right reed switch for the job can be a matter of trial and error, says Hauser. The correct length, amp draw, air gap and sensitivity, mounting requirements and other factors all come into place.

“It isn’t like a toggle switch,” he says, “the engineers of both Hasco (and other vendors) and the end customer can work together for days, weeks and months, depending on the complexity of the design,” says Hauser. For one customer, it took months to develop the reed switch.

It is not only choosing the correct reed switch that is important, but also determining the correct actuating positions. Berns says that there are several standard and recommended actuating positions and many alternative positions. These may include an actuating position for a perpendicular motion in which the switch only closes once with each magnet pass, keeping the switch closed for the maximum magnet travel. Another alternative is a parallel motion that provides as many as three closures with the maximum magnet travel, but can provide only one closure with restricted movement. A front-to-back motion is similar to parallel motion, except that the motion is at right angles to the switch and it completely passes by the switch.

“The applications can be like art in a way,” says Berns. “Sometimes people will come to us with an application and nobody makes anything that will work. And we will help them develop a reed switch and magnet for the application.”

For more information, email:
Coto Technology: sales@cotorelay.com
Hamlin Electronics: simon.pitkin@hamlin.com
Hasco Components: tabh@hascorelays.com
MEDER Electronic: rgreenberg@meder.com
Reed Switch Developments: rberns@reedswitchdevelopments.com