Appliance Design - August 2007 - (Page 15)

SWITCHES & RELAYS 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. In another Meder application, reed sensors applianceDESIGN August 2007 15 The leads on this Hamlin Flex reed switch can be bent to fit. 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. Reed switches from Hamlin. www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - August 2007

Contents Editorial Shipments/Forecasts News Watch A Reed Switch is Simople, yet Versatile, and Designers are Finding New Uses for this Time-Tested Technology A Novel Actuation and Sensing Technology can Register a Light Touch, but Resist Blows from a Hammer Tracing the Source of Noise is Made Easier with Noise Source Identification Methods Appliance Noise can be Annoying, but Designers have Many Material Choices to Help Rein in the Racket Nanocomposites are No Longer a Technology for the Future. These Plastic Materials are Already on the Job A New High-Strength, Low-Density Glass-Microsphere Additive can Improve Plastic Part and Molding Performance A New Electromagnetic Weld/Bonding Process Helps Ensure Long-Term Integrity of Joints in Plastic Parts Snap-Based Technology Eases Installation and Makes Products More Serviceable Two-Part Silicone Adhesives Provide a Rapid-Cure Alternative to Traditional One-Part Silicones Classified DesignMart Advertiser’s Index Association Report: GAMA

Appliance Design - August 2007

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