Sensing technology could render the reed switch obsolete

Given their ubiquity, then, when a company comes forward with the claim of having developed a technology that it believes will render reed switches obsolete, one is bound to sit up and take notice. That, however, is what Honeywell Sensing and Control is claiming for its new Nanopower Magnetoresistive Sensor Integrated Circuits. Honeywell's Nanopower Anisotropic Magnetoresistive Sensor ICs that provide the highest level of magnetic sensitivity (as low as 7 Gauss typical) while requiring nanopower (360 nA). When compared to other, widely-used magnetic technologies, these sensors offer design engineers a number of advantages. Based on a combination of AMR and BiCMOS technology, these sensors are smaller and more durable and reliable than reed switches, at the same sensitivity and essentially the same cost, the new Nanopower Series Magnetoresistive Sensor ICs are ideal for battery-powered applications where previously only reed switches could be used due to very low power requirements and large air gap needs. The drivers for their development came from a number of factors affecting the sensor industry. These included a desire for miniaturised sensors for battery-powered operation that were not over-reliant on the increasingly expensive rare metals needed for permanent magnets. Equally, there was a desire for low power consumption, smaller size and high Gauss Sensitivity. While Reed switches offered low power consumption and high sensitivity, they are too big for many applications. Equally, Hall Effect sensors are insufficiently sensitive, while the power consumption of AMR sensors leaves something to be desired. Compared with Hall-effect sensors, the new Nanopower Series Magnetoresistive Sensor ICs' higher sensitivity can offer the ability to sense air gaps over twice the distance of Hall-effect sensors. The higher sensitivity improves design flexibility and can offer significant application cost savings by using smaller or lower strength magnets. James McKenna, Honeywell Sensing & Control's product director, electronic sensing, says of the Nanopower Series: "We feel that this technology has the potential to render reed switches obsolete in time. The sensitivity of these sensors is ten times greater than AMR sensors, they require much smaller magnets, thus reducing cost, they are durable, solid-state sensors, which makes them much more durable compared to the glass tubes in which reed switches must be contained. Equally, unlike reed switches, they do not break or wear out over time." However, it is not solely in comparison to reed switches that McKenna believes the Nanopower Series can succeed, but in opening up entirely new applications. "The main point to bring across," he says, "is that reed switches are now at the limit of what they can achieve, so nanopower allows designers to go that bit further. With these highly-sensitive, durable, small and reliable sensors we can now realise applications that were simply not possible before." The Nanopower Series Magnetoresistive Sensor ICs are designed for use in a wide range of battery-operated applications including water and gas meters, electricity meters, industrial smoke detectors, exercise equipment, security systems, handheld computers, scanners, as well as white goods such as dishwashers, microwaves, washing machines, refrigerators and coffee machines, and medical equipment such as hospital beds, medication dispensing cabinets, infusion pumps, and consumer electronics such as notebook computers, tablets, and cordless speakers. In the industrial sphere, for instance, these sensors may be used to detect if the lid of battery-operated equipment is open or closed. Alternatively, they can be used as a counter in water and gas meters to determine water or gas use. At the moment, these applications require reed switches but, because of the low power requirements (500 nanoamps) of the Honeywell system can meet the need for lengthy battery life. In fact, these sensor ICs use a very low average current consumption and a push-pull output that does not require a pull-up resistor. The sensor ICs can operate from a supply voltage as low as 1.65V. In this context of water, electric and gas utility meters, the Nanopower series also offers particular benefits as it can be used to counter tampering. This is possible because they can be used to detect the presence of a magnetic field applied to the meter by a large external magnet in order to tamper with, slow down or even stop the meter's counting function. "Due to the significant price increases for rare earth magnets, design engineers using Hall-effect sensors have been looking for ways to decrease the total cost of design by using less magnetic material, or moving to a more common magnet in their applications," says Josh Edberg, senior product marketing manager for Honeywell Sensing and Control. "Design engineers are also looking for an alternative to reed switches to reduce size and increase quality and durability, while maximising battery life. Honeywell's new Nanopower Series Magnetoresistive Sensor ICs are ideal for these battery-powered applications due to their high sensitivity and nanopower." The Nanopower Series is available in two magnetic sensitivities. The first is the ultra-high sensitivity SM351LT (7 Gauss typical operate; 11 Gauss maximum operate; very low current draw (360 nA typical). The other is the very high sensitivity SM353LT (14 Gauss typical operate; 20 Gauss maximum operate; very low current draw, 310 nA typical). The push-pull (CMOS) output does not require external resistors, making it easier and more cost-effective to operate. The non-chopper stabilised design eliminates electrical noise generated by the sensor. The subminiature SOT-23 surface mount package, supplied on tape and reel (3000 units per reel), is smaller than most reed switches, allowing for use in automated pick-and-place component installation as well as greatly reduced production costs.