Practical Sensors: The Hall Effect

Measuring a magnetic field can be very easy with some pretty low tech, or it can be very high tech. It just depends on what kind of measurement you need and how much effort you want to expend. The very simplest magnetic sensors are reed switches. These are basically relays with no coil. Instead of a coil, an external magnet gets close enough to make or break the contacts in the reed. You see these a lot in, for example, door alarm sensors.

Then again, there’s no real finesse to a reed. It changes state when it sees enough of a magnetic field and that’s about all. You could use a compass with some sort of detection on the needle to get some more information about the field, but not much more. That was, however, how early magnetometers worked. Today, you have lots of options, including the nearly ubiquitous Hall effect sensor.

You might use a Hall effect to measure the magnetic button on a keyboard key coming down when you press it or the open and closed state of a valve. A lot of Hall effects see service as current monitors. Since a coil generates a magnetic field proportional to the current through it, a magnetic sensor can estimate the current in a coil of wire without any physical contact. Hall effects can also watch a magnet go by in a linear motion system or a rotating system to get an idea of position or speed. For example, check out this brushless motor controller that uses three sensors to understand the motor’s position.

History

Edwin Hall identified the effect in 1879. The basic idea is simple: an electrical conductor carrying current will exhibit changes due to an external magnetic field nearby. These changes show up as voltage you measure across the conductor. Normally, the voltage across a conductor will be nearly zero, but with a magnetic field, you’ll get a non-zero reading in proportion to the magnetic field strength in a particular plane, as we’ll see shortly.

Hall effect sensors are just one type of modern magnetometer. There are many different kinds including those that use inductive pickup coils that may or may not rotate or a fluxgate, which is a special type of coil. Some use a scale or a spring to measure force against another magnet — sometimes microscopically. You can even detect a magnetic field using optical properties like the Kerr effect or Faraday rotation.

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Hamster Goes On Virtual Journey

Hamsters are great pets, especially for those with limited space or other resources. They are fun playful animals that are fairly easy to keep, and are entertaining to boot. [Kim]’s hamster, [Mr. Fluffbutt], certainly fits this mold as well but [Kim] wanted something a little beyond the confines of the habitat and exercise wheel and decided to send him on a virtual journey every time he goes for a run.

The virtual hamster journey is built on an ESP32 microcontroller which monitors the revolutions of the hamster wheel via a hall effect sensor and magnet. It then extrapolates the distance the hamster has run and sends the data to a Raspberry Pi which hosts a MQTT and Node.js server. From there, it maps out an equivalent route according to a predefined GPX route and updates that information live. The hamster follows the route, in effect, every time it runs on the wheel. [Mr Fluffbutt] has made it from the Netherlands to southeastern Germany so far, well on his way to his ancestral home of Syria.

This project is a great way to add a sort of augmented reality to a pet hamster, in a similar way that we’ve seen self-driving fish tanks. Adding a Google Streetview monitor to the hamster habitat would be an interesting addition as well, but for now we’re satisfied seeing the incredible journey that [Mr Fluffbutt] has been on so far.

Street-Legalize Your Ebike With A Magnet

Getting into e-biking is a great hobby. It can get people on bikes who might otherwise not be physically able to ride, it can speed up commute times, and it can even make hauling lots of stuff possible and easy, not to mention it’s also fun and rewarding. That being said, there are a wide array of conflicting laws around what your e-bike can and can’t do on the road and if you don’t want to run afoul of the rules you may need a programmable device that ensures your e-bike is restricted in the appropriate way.

This build is specifically for Bafang mid drives, which can be up to 1000 W and easily power a bike beyond the speed limit where [Tomblarom] lives. A small microcontroller is housed in a waterproof box on the bike and wired between the motor’s display and controller. A small hall effect sensor and magnet sit by this microcontroller, and if the magnet is removed then the microcontroller reprograms the bike’s controller to limit the speed and also to disable the throttle, another feature that is illegal in some jurisdictions but not others. As an added bonus, the microcontroller also handles brake lights, turn signals, and automatic headlights for the bike as well.

While the project page mentions removing the magnet while getting pulled over to avoid fines and other punishments, that’s on you. We imagine this could still be useful for someone who wants to comply with local laws when riding on the road, but still wants to remove the restrictions when riding on private property or off-road where the wattage and speed restrictions might not apply.

Simple Sensor Makes Filament Measurements A Snap

Just how tight are the manufacturing tolerances of modern FDM printer filament. Inquiring minds want to know, and when such minds are attached to handy fellows like [Thomas Sanladerer], you end up with something like this home-brew filament measurement rig to gather the data you seek.

The heart of this build is not, as one might assume, some exotic laser device to measure the diameter of filament optically. Those exist, but they are expensive bits of kit that are best left to the manufacturers, who use them on their production lines to make sure filament meets their specs. Rather, [Thomas] used a very clever homemade device, which relies on a Hall effect sensor and a magnet on a lever to do the job. The lever is attached to a roller bearing that rides on the filament as it spools through the sensor; variations in diameter are amplified by the lever arm, which wiggles a magnet over the Hall sensor, resulting in a signal proportional to filament diameter.

The full test rig has a motor-driven feed and takeup spools, and three sensors measuring across the filament in three different spots around the radius; the measurements are averaged together to account for any small-scale irregularities. [Thomas] ran several different spools representing different manufacturers and materials through the machine; we won’t spoil the results in the video below, but suffice it to say you probably have little to worry about if you buy from a reputable vendor.

When we see a filament sensor, it’s generally more of the “there/not there” variety to prevent a printer from blindly carrying on once the reel is spent. We’ve seen a few of those before, but this is a neat twist on that concept.

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3D-Printed Flight Controls Use Magnets For Enhanced Flight Simulator 2020 Experience

We have seen quite a few DIY joystick designs that use Hall effect sensors, but [Akaki Kuumeri]’s controller designs (YouTube video, embedded below) really make the most of 3D printing to avoid the need for any other type of fabrication. He’s been busy using them to enhance his Microsoft Flight Simulator 2020 experience, and shares not just his joystick design, but makes it a three-pack with designs for throttle and pedals as well.

Hall effect sensors output a voltage that varies in proportion to the presence of a magnetic field, which is typically provided by a nearby magnet. By mounting sensors and magnets in a way that varies the distance between them depending on how a control is moved, position can be sensed and communicated to a host computer.

In [Akaki]’s case, that communication is done with an Arduino Pro Micro (with ATmega32U4) whose built-in USB support allows it to be configured and recognized as a USB input device. The rest is just tweaking the physical layouts and getting spring or elastic tension right. You can see it all work in the video below.

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TMD-1 Makes Turing Machine Concepts Easy To Understand

For something that has been around since the 1930s and is so foundational to computer science, you’d think that the Turing machine, an abstraction for mechanical computation, would be easily understood. Making the abstract concepts easy to understand is what this Turing machine demonstrator aims to do.

The TMD-1 is a project that’s something of a departure from [Michael Gardi]’s usual fare, which has mostly been carefully crafted recreations of artifacts from the early days of computer history, like the Minivac 601  trainer and the DEC H-500 computer lab. The TMD-1 is, rather, a device that makes the principles of a Turing machine more concrete. To represent the concept of the “tape”, [Mike] used eight servo-controlled flip tiles. The “head” of the machine conceptually moves along the tape, its current position indicated by a lighted arrow while reading the status of the cell above it by polling the position of the servo.

Below the tape and head panel is the finite state machine through which the TMD-1 is programmed. [Mike] limited the machine to three states and four transitions three symbols, each of which is programmed by placing 3D-printed tiles on a matrix. Magnets were inserted into cavities during printing; Hall Effect sensors in the PCB below the matrix read the pattern of magnets to determine which tiles are where. The video below shows the TMD-1 counting from 0 to 10, which is enough to demonstrate the basics of Turing machines.

It’s hard not to comment on the irony of a Turing machine being run by an Arduino, but given that [Mike]’s goal was to make abstract concepts easy to understand, it makes perfect sense to leverage the platform rather than try to do this with discrete logic. And you can’t argue with results — TMD-1 made Turing machines clear to us for the first time.

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Laundry Monitor Won’t Generate Static With Roommates

Laundry. It’s one of life’s inescapable cycles, but at least we have machines now. The downside of this innovation is that since we no longer monitor every step — the rock-beating, the river-rinsing, the line-hanging and -retrieving — the pain of laundry has evolved into the monotony of monitoring the robots’ work.

[Adam] shares his wash-bots with roommates, and they aren’t close enough to combine their lights and darks and turn it into a group activity. They needed an easy way to tell when the machines are done running, and whose stuff is even in there in the first place, so [Adam] built a laundry machine monitor that uses current sensing to detect when the machines are done running and sends a text to the appropriate person.

Each machine has a little Hall effect-sensing module that’s carefully zip-tied around its power cable. The signal from these three-wire boards goes high when the machine is running and low when it’s not. At the beginning of the load, the launderer simply presses their assigned button on the control box, and the ESP32 inside takes care of the rest.

Getting a text when your drawers are clean is about as private as it gets. Clean underwear, don’t care? Put it on a scrolling marquee.