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.


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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A Magnetic Field Strength Meter Using An Arduino

We’re used to Hall effect devices as proximity sensors in mechanical systems, used to provide detection of something that has a magnet attached to it. However it’s easy to forget that the devices that provide a magnet-or-not digital output are only part of the story, and linear Hall effect devices provide a handy way to measure a static magnetic field. It’s something [mircemk] demonstrates, with an Arduino-powered magnetic field strength meter that uses a UGN 3503U Hall effect device.

The circuit is extremely simple, comprising the sensor, an Arduino Nano, and an OLED display. This device is handy because its voltage output has a known relationship to the gauss level the sensor is experiencing, so while the accuracy of its calibration isn’t verified it can at least give a believable reading derived from the Arduino’s ADC.

The whole is wrapped up in an attractive case that looks as though it has been made from PCB material, with the sensor protruding on what seems to be the shell of a plastic ballpoint pen. It makes a handy instrument that provides a useful function for not a lot of money, so what’s not to like! Take a look at the video below the break for the full story.

Surprisingly such projects are few and far between here at Hackaday, however it’s not the first magnetic field measurement we’ve seen.

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Fire In The Palm Of Your Hand

For as long as super-heroes have existed, they have inspired hacker projects. For [Everett Bradford], emulating the character Pyro from X-Men has been an on and off project for the last decade. His latest version, Pyro System V4, integrates quite a bit of control electronics to give the rather convincing effect of mind-controlled fire in the palm of his hand. (Video, embedded below.)

The system is a motor-actuated slider strapped to [Everett]’s forearm, which pushes a pivoting end-effector with an integrated butane burner into the palm of his hand. The slider runs on 4 mm linear bearings actuated by a small geared DC motor using cables. The end effector is spring-loaded to push it into the palm and integrates a high voltage ignition arc generator circuit, nozzle, and capacitive activation button.

The butane gas canister and the valve was cannibalized from a small blow torch lighter, and the valve is actuated by another geared DC motor. The valve actuator, slide actuator, and end-effector hinge all integrate position feedback via hall effect sensors and magnets. The sensor in the hinge allows the slide to actively correct for the angle of the user’s wrist, keeping the end effector in the middle of the palm.

The control circuit is split into two parts. One PIC16 microcontroller runs all the motion control and position sensing, while a PIC18 connected to a small touch screen handles user interface, control parameters, and ignition. The touch screen proved especially useful for control parameters during development without needing to connect to a laptop.

Some of [Everett]’s previous version had a much more impressive (and dangerous) flame but was also very bulky. We think this latest version strikes a pretty good balance regarding compactness and achieving convincing illusion.

[Colin Furze] is another name commonly associated with fire-breathing contraptions, but they have a proven history of landing him in hospital.

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An Alternator Powered Electric Bicycle Gives Rotor Magnetic Field Insight

For anyone involved in the construction of small electric vehicles it has become a matter of great interest that a cheap high-power electric motor can be made from a humble car alternator. It’s a conversion made possible by the advent of affordable three-phase motor controllers, and it’s well showcased by [austiwawa]’s electric bicycle build video (embedded below).

The bike itself is a straightforward conversion in which the motor powers the rear wheel via an extra sprocket. He tried a centrifugal clutch with limited success, but removed it for the final version. Where the interest lies in this build is in his examination of Hall effect sensor placement.

Most alternator conversions work without sensors, though for better control it’s worth adding these magnetic sensors to allow the controller to more directly sense the rotation. He initially placed them at the top of the stator coils and found them to be ineffectual, with the big discovery coming when he looked at the rotor. The electromagnet in the rotor on a car alternator has triangular poles with the field concentrated in the centre of the stator, thus a move of the sensors to half way down the stator solved the problem. Something to note, for anyone converting an alternator.

Should you wish to give it a try, a year ago we published a primer on turning car parts into motors.

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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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Useless Machine For An Existential Quandary

There’s no project that dives into existential quandaries more than a useless machine, as they can truly illustrate the futility of existence by turning themselves off once they have been powered on. Typically this is done with a simple switch, but for something that can truly put the lights out, and then re-illuminate them, [James]’s latest project is a useless machine that performs this exercise with a candle.

The project consists of two arms mounted on a set of gears. One arm has a lighter on it, and the other has a snuffer mounted to a servo motor. As the gears rotate, the lighter gets closer to the candle wick and lights it, then the entire assembly rotates back so the snuffer can extinguish the flame. Everything is built around an Arduino Nano, a motor driver powering a Pitman gear motor, and a set of Hall effect sensors which provide position data back to the microcontroller.

If you’re in the mood for a little existential angst in your own home, [James] has made the project files available on his GitHub page. We always appreciate a useless machine around here, especially a unique design like this one, and one which could easily make one recognize the futility of lighting a candle at all.

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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.