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.)



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
Jun 22, 2021
Have you ever been in a situation where the run has started and you realize that you needed to add two more workers, or drop a couple of them? In such cases, you wait for the run to complete, make... [[ Click on the title to access the full blog on the Cadence Community site...
Jun 21, 2021
By James Paris Last Saturday was my son's birthday and we had many things to… The post Time is money'¦so why waste it on bad data? appeared first on Design with Calibre....
Jun 17, 2021
Learn how cloud-based SoC design and functional verification systems such as ZeBu Cloud accelerate networking SoC readiness across both hardware & software. The post The Quest for the Most Advanced Networking SoC: Achieving Breakthrough Verification Efficiency with Clou...
Jun 17, 2021
In today’s blog episode, we would like to introduce our newest White Paper: “System and Component qualifications of VPX solutions, Create a novel, low-cost, easy to build, high reliability test platform for VPX modules“. Over the past year, Samtec has worked...

featured video

Reduce Analog and Mixed-Signal Design Risk with a Unified Design and Simulation Solution

Sponsored by Cadence Design Systems

Learn how you can reduce your cost and risk with the Virtuoso and Spectre unified analog and mixed-signal design and simulation solution, offering accuracy, capacity, and high performance.

Click here for more information about Spectre FX Simulator

featured paper

4 common questions when isolating signal and power

Sponsored by Texas Instruments

A high-voltage circuit design requires isolation to protect human operators, enable communication to lower-voltage circuitry and eliminate unwanted noise within the system. Many options are available when designing a power supply for digitally isolated circuits including; flyback, H-bridge LLC, push-pull, and integrated isolated data and power solutions. This article explores common questions when isolating signal and power in a design as well as a brief overview of available power solutions.

Click to read more

featured chalk talk

SN1000 SmartNIC

Sponsored by Xilinx

Cloud providers face a variety of challenges with moving data from one place to another. In modern data centers, flexibility is a key consideration - on par with performance. Software-defined hardware acceleration offers a major breakthrough in flexibility. In this episode of Chalk Talk, Amelia Dalton chats with Kartik Srinivasan of Xilinx about the details of Smart NICs with the new Alveo SN1000 with composable hardware.

Click here for more information about the Alveo SN1000 - The Composable SmartNIC