Magnetic speed sensor

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.

Comments

[mandrews4]

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 ?

[shodanx]

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)

[PeterTheUnGreat]

Hi,

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
Pete
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?

[dresch]

Thanks! Really nice job. A great alternative for rigging up a quick custom sensor. Excellent.

[luslugger]

I would like to rotate the magnet and see the voltage rise and fall as it passes the coil. Would I just need a coil? Is there a simple way to get an iron core to wind the wire on? How does the coil winding orient to the magnet? If I need a 10 volt output do I put on more turns or get a stronger magnet? Both?

[Wyle_E]

A steel bolt will do. Radio Shack sels (or used to; it's been a while) magnet wire in small spools. Magnet wire has thin enamel insulation that lets you pack more turns into a given space, but any insulated wire will work. You're not trying to generate significant current, so smaller wire will let you use more turns. Note that small coin-shaped magnets are usually have their poles on the flats, so you need to spin the magnet like a flipped coin. The voltage you get is proportioal to the product of the strength of the magnetic field, the number of coil turns and the speed of the coil through the field. Depending on your application, you may be able to save some time by driving a small permanent-magnet motor as a generator.

[nafango22]

i'm lovin the old school game boy in the back of this picture

[mje]

Nice- although there are easier sources of magnets. You could probably sell the phone for $20 and buy a ton of small neodymium magnets!