editor's blog
Subscribe Now

Sensor or Switch?

Honeywell recently released a new AMR (anisotropic magneto-resistive) sensor. We looked at this basic technology some time back, but there was another aspect of the release that confused me: the sensor was compared to a reed switch. And, at first glance, I don’t see a switch (=actuator) and a sensor as being the same thing.

For those of you steeped in this technology, what follows may seem rather basic and even obvious. But if you’re new to the space, then there’s some room to untangle some concepts that can be easily conflated.

Part of the issue has to do with being precise with terms that might be confused. If I think sloppily, I end up confusing a reed switch with a reed relay. What’s the difference? Well, a reed switch is simply a two-lead component. The switch connects the leads, presumably completing some circuit. That switch is actuated by a magnetic field (either to open or close it). That field is applied externally; exactly how depends on the application. Critically, there’s no magnetic component built into the switch.

So, in a way, the reed switch is a magnetic field detector. When the field exceeds a threshold, the reed moves, and you can think of this as a crude digital magnetic field sensor.

Now, if you include a magnetic coil along with the reed switch, adding two new leads, now you have a reed relay. This is much more of an actuator than a sensor, since it creates its own magnetic field. So switch and relay confusion can create sensor and actuator confusion.

Now let’s look at the AMR sensor schematic from the data sheet. From the outside, it may look just like a Hall Effect sensor, another sensor based on magnetic phenomena. (The field directions are apparently different, but I won’t dwell on that.)

Figure.png

 

On the left is the detector circuit. Because this constantly draws power, it must do so exceedingly sparingly. The original application for this (more on that in a moment) required no more than 500 nA; Honeywell has a couple of devices, one at 310 nA, the other at 360 nA. They claim this to be more than an order of magnitude more miserly than the lowest-power Hall Effect device, with greater sensitivity.

Once it detects the field, it flips the flop and the output value changes. Now… this output looks something like a beefy CMOS output, not like a wire in a reed switch. And if it drives a CMOS input, then this will simply look like a digital indicator with no DC load current. But if the output drives something that pulls current, then the pull-up (or the pull-down) acts as a switch that makes or breaks that circuit. In this way it more resembles a reed switch.

Here’s one other possible source of significant confusion: this is not like the magnetometer you may have in your phone. Your phone mag, like most sensors, provides continuous readings of the ambient magnetic environment. The phone can go in and interrogate the value at any time. By contrast, this AMR sensor is digital: either on or off. You can’t go in and measure the actual field. So it’s unlike many other sensors out there. That on/off characteristic is what makes it appear to be a switch – and contributes to the sensor/switch confusion.

So if you think of a reed switch as a switch that can be used as a sensor, then here you have a mag sensor that can be used as a switch.

By the way, that application I alluded to above? Apparently people were trying to monkey with electric meters using magnets to disrupt the metering. So AMR sensors (it takes two of them) are used to detect such anomalous magnets. Obviously, being in a meter, they have access to power, but it’s the power someone else is paying for, so it has to be tiny so as to be undetectable on their bill.

You can read more about Honeywell’s part in their release.

Leave a Reply

featured blogs
Sep 21, 2020
Technology is changing the strategies we use to do things - oh so fast that 2010 seems like a distant past- within many spaces -- including the way we do our current topic of interest - Timing... [[ Click on the title to access the full blog on the Cadence Community site. ]]...
Sep 21, 2020
Semicon, the world’s largest semiconductor conference and exhibition, is September 23-25 in Taiwan. Like most shows of its size and caliber, Semicon boasts a long and illustrious list of exhibitors (500+), and countless forums, symposiums, and workshops. Of course Semic...
Sep 18, 2020
[From the last episode: We put the various pieces of a memory together to show the whole thing.] Before we finally turn our memory discussion into an AI discussion, let'€™s take on one annoying little detail that I'€™ve referred to a few times, but have kept putting off. ...
Sep 16, 2020
In addition to the Great Highland (Scottish) bagpipes, the Uilleann (Irish) bagpipes, and the Northumbrian (English) bagpipes, there are myriad other offerings spanning the globe....

Featured Video

Four Ways to Improve Verification Performance and Throughput

Sponsored by Cadence Design Systems

Learn how to address your growing verification needs. Hear how Cadence Xcelium™ Logic Simulation improves your design’s performance and throughput: improving single-core engine performance, leveraging multi-core simulation, new features, and machine learning-optimized regression technology for up to 5X faster regressions.

Click here for more information about Xcelium Logic Simulation

Featured Paper

Helping physicians achieve faster, more accurate patient diagnoses with molecular test technology

Sponsored by Texas Instruments

Point-of-care molecular diagnostics (PoC) help physicians achieve faster, more accurate patient diagnoses and treatment decisions. This article breaks down how molecular test technology works and the building blocks for a PoC molecular diagnostics analyzer sensor front end system.

Read the Article

Featured Chalk Talk

ERFV Coax Connectors

Sponsored by Mouser Electronics and TE Connectivity

5G pushes every dimension of electronic and RF design, and that puts extraordinary demand on or connectors. The best designs in the world won’t work reliably if your connector solution isn’t up to the task. In this episode of Chalk Talk, Amelia Dalton chats with Claude de Lorraine of TE Connectivity about ERFV Coax Connectors - RF connectors that are designed specifically for 5G applications.

Click here for more information about TE Connectivity ERFV Coax Connectors