For decades electronic devices have been “turned on” using some type of mechanical power switch. Whether it was a slider, a toggle, or a push-button switch, the method for powering on the electronic device was some type of manually operated switch with mechanical contacts. But with the massive increase in electronic devices that are battery operated, small, wearable, and ingress protected, all with a desire to make them simple and user friendly, product designers are looking at alternatives to the mechanical switch.
For battery-operated devices, you want to keep the electronics’ power off (or at least in a sleep state) until the device must be operational. While mechanical switches are relatively simple and power efficient (very high resistance in the off state) they have several disadvantages.
First, mechanical power switches are not optimal for products that need to be ingress protected. Second, many devices are so small that there’s not enough space for a mechanical switch. Lastly, while turning the device on would seem to be intuitive, customer support agents can relate an amazing number of support issues resulting from users forgetting to turn the device on.
In looking for alternatives, product designers often consider wireless turn on, especially if the product, like many IoT products, are already wirelessly enabled. Wireless interfaces don’t present an ingress protection challenge, and an end user may have to configure the device wirelessly so turning it on wirelessly isn’t a burden. But the biggest drawback to this method is that it requires the devices radio receiver to remain powered on.
An increasingly popular alternative for turning on such devices is a magnetic turn on, using a magnetic sensor inside the device which senses when a magnetic field that has been introduced or removed. That field is generated from a magnet that’s either embedded in the packaging or in a secondary component. Such components include an applicator or a magnet that the user can “swipe” to power up the device.
Because this process is contactless, and magnetic fields move easily through plastics, magnetic sensing is compatible with the ingress protection goal. This “power on” method isn’t new and designers have been using reed switches as a magnetic sensor for decades. Passive reed switches “close” when a field is introduced, and “open” when it’s removed. As such, they are essentially a form of mechanical switch but operated magnetically instead of manually and also quite power efficient. But reed switches can be relatively large, where the end devices themselves are small.
Solid-state magnetic sensors, such as hall sensors, have been around for decades, and they’re smaller and more reliable than reed switches. But, they consume a relatively large amount of power.
A third type of magnetic sensor, based on tunneling magneto-resistive (TMR) technology, has moved to forefront of magnetic sensing due to its low current consumption, as low as 100 nA on average. These magnetic sensors are offered in packages as small as an LGA-4. They meet the requirements for size, power efficiency, ingress protection, and user friendliness. You’ll find these TMR magnetic sensors in wearable, implantable, and ingestible medical devices, as well as a host of other small, battery operated, and ingress protected IoT devices.
About the Author
As Global Business Development Manager for Coto Technology, Tim works closely with solution providers in industrial, consumer, medical and commercial industries to address their magnetic sensing needs. Tim has also worked for Analog Devices and other embedded technology suppliers in product marketing and business development roles.