Magnetic Speed Sensor





Introduction: Magnetic Speed Sensor

About: I am Married to Beth, I am an Architect and have four wonderful children

How to make a magnetic speed sensor similar to those used to sense wheel speed in ABS systems

Step 1: A Magnetic Speed Sensor Is...

The sensor works using a iron cored coil with a magnet attached to one end. When a peice of ferrous metal is moved towards the end of the sensor it changes the shape of the magnetic field in the coil, this changing magnetic field then induces current to flow in the windings of the coil resulting in a small amount of electricity being generated.
This sensor only detects movement of ferrus objects near the sensor so is typically used for speed sensing (for example a wheel sensor in an ABS system) usually in combination with a toothed steel wheel.
They are very rugged sensors and are not affected by dirt and have a very high signal output making them less sensitive to noise (ideal for automotive applications)
For example the sensor I have made will generate a sine wave of 50Vp-p into a 1Kohm load (that is a peak of 50mV)

Step 2: You Will Need the Following...

1. A large nut and bolt
2. A relay (I used a miniature 12V relay, but others should work)
3. A broken mobile phone
4. A bit of 2 core wire (twisted pair will help with noise immunity, but is not necessary)

Step 3: Get the Magnet

Rip apart the mobile phone and recover the earpiece, rip this apart and you will be left with a small but strong magnet.

Step 4: Get the Coil

Now dissasemble the relay very carfully (it is very easy to break the coil wires) until you are left with just the coil and it's iron core.

Step 5: Attach the Magnet

Place the magnet on the end of the core of the coil, there is usually a 'head' to the core a bit like a nail - stick the magnet on this end.
Make sure the magnet is at the same end as the connections for the coil

Step 6: Attach the Wire

Solder the wire onto the coil contacts

Step 7: Case the Sensor

Now drill the centre out of the bolt and fit the sensor inside using plenty of hotmelt glue (or epoxy).
You should end up with the very tip of the iron core of the coil showing at one end as in the photo.
Make sure the connections inside don't touch the case, prehaps a peice of plastic tube might be usefull as an insulator.

Step 8: Test the Sensor

Here I have a test rig lashed up on my bench - a gear from my lathe mounted in the chuck of an electric drill with a 'G' clamp on the trigger.
The sensor is then connected to my scope.

Step 9: Result!

Here you see the sensor working, the closer you can get the sensor to the moving teeth, the larger the output.
Here the scope is set to 5V per division so it is outputting about 40Vp-p (Although this is with no load as I was in too much of a rush this morning and forgot to put on a load resistor - If I had used a 1K resistor the output would have probably dropped to about 5V)

If you can be very patient I will post some circuits in the future to use this in real life, I.e. attached to my Landrover engine.



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    Hey Peter, nice work!

    I thought I'd seen this technology used in drag racing. They used a sensor like that to read driveshaft speed which in turn helped ID tire spin. It's traction control when wired to retard engine timing for fractions of a second. I haven't heard much recently or seen it up close. Anything interesting there for you? Sadly, my hotrod doesn't have the problem of removing power to reduce out of control tire spin. Not yet anyhow

    This is very similar to crank position sensors, wouldn't you think?



    What sort of landy you have? Mine is an old range rover with a faulty speedo, and I'd be very glad to install an alternative to know what's my current speed.

    First I thought of fixing up a hall sensor somewhere close to the transmission brake drum and putting a small magnet just on top of that drum, but that might cause some disbalance and it would show me just the speed of vehicle, while setting up four bolts like yours would allow me to track the speed of each wheel - I could even vizualize the traction on a pda or an ipod :-)

    Right now I am investigating how the ABS/TC sensors are installed on the newer landies. I'd be glad to see continuation of your post - have you installed your sensor?

    2 replies

    Sadly I had to part with my Landrover before I got any further with the project. (My first child turned up - now just turning 4 on Monday!)
    You will have to take on the mantle and post an instructable showing the sensor in action.

    can I see the circuit to use in real life?

    Nice project.
    I'd like to see a basic circuit to check speed without needing to tote an oscilliscope around.
    Is there a simple, cheap, robust, mobile, lightweight circuit that would ride on my bike, for instance? Would this sensor be sensitive enough to detect a bicycle spoke?

    what would i do if i wanted it to detect a magnet. i lost the sensor for my bicycle speedo

    i would love to pick your brain over how you are going to mount this on your engine because i need a speed sensor on my 73 v8 dodge honey bee so i can add cruse control

    4 replies

    Hi, I'm not Peter the ungreat but I have a suggestion. take it or leave it. AutoMeter makes a digital speedo kit which comes with a modern sensor that you could "piggyback" to get your cruise input from. (you would lose your stock speedo functionality though.) You could use a modern rear diff/axle assy with an ABS sensor. This could be the input for cruise, and you would still have the cable op speedo. You could modify the oem gear to fit a modern speed sensor that might fit the original location on the extension housing of your trans, again this would make the oem speedo inop. Last but not least.... Old Cruise control units were vacuum operated, find one of your vintage and make it work. I hope this helps.

    I actually found a old dodge motor home in the junk yard while harvesting a carb i pulled the cruse control and its running much better now except the cab over bed needs work

    Nice!" well, except for the fact that bodywork isn't any fun. " best of luck to you.

    ya well it turns out that there was water damage in the frame and a crunch with a branch smashed it in. to bad but6 its tarped off waiting for the snow to go

    Fantastic! I love it, some nice bit of magic there.

    I have a tricky question. I want the sensor to produce a signal, which goes to a guitar bass amplifier, and produces a thump, when metal comes close to it.

    How do I make the sensor produce a signal around the 33-100 hz range ?

    is there a limit to the maximum signal frequency ?

    I'd like to make a sensor like this to measure the six sided nut from a turbocharger shaft with a top speed of about 120 Krpm

    not sure if I should go with an optical sensor instead (also not perfect because of condensation in impeller inlet blocking the view)

    1 reply


    I have never tried to find the upper frequency limit, but I guess it is limited by the iron core - I know that when you start making high frequency transformers you need to use dust cores rather than iron. although having said that it is the change in flux through the windings that matters.
    My best advice is give it a go. To test I suggest using a large diameter toothed wheel running at a lower speed.
    This sort of sensor is very good in a dirty environment - however I know that rare earth magnets are limited by temperature (about 90 degrees Celsius) and they start to loose their magnetism.
    Hope that helps
    P.S. could you use an audio sensor with a high pass filter as nothing else in the engine will emit the frequency that the turbo will?

    How do i put the magnet in between the coil and the iron...there is no gap there...any workaround??

    1 reply

    The magnet is just stuck to the end of the nail using it's own magnetism. Stick the magnet on the non-sensing end. (See step 5)

    Do we have to fit the sensor inside the bolt???If yes..why??

    Does the sensor work directly without enclosing it in the bolt...Please reply urgently...

    1 reply

    Yes, the sensor works fine un-enclosed. The bold is just to make the whole sensor rugged and easy to mount securely.
    This type of sensor is often fitted in very hostile environments (I.e. near the road on a car - hence needing to be very rugged)

    Do we have to fit the sensor inside the bolt???If yes..why??

    Does the sensor work directly without enclosing it in the bolt...Please reply urgently...