Spindle Speed Sensor From a Guitar Tuner




Introduction: Spindle Speed Sensor From a Guitar Tuner

About: I'd rather be efficient than professional.

I just got a Pro membership a couple of days ago, thanks to Random_Canadian's generosity. (Thanks for the hookup!) I figured it was time to quit lurking and actually contribute, so here goes my first Instructable. 

As a CNC technician, there have been a few occasions where I've wondered if a spindle or motor shaft was spinning as fast as the motor controller claimed. An old trick I learned a long time ago involved a digital guitar tuner. The trick was to hold a piece of stiff metal shim stock, around .020" or so thick, against a protrusion on the shaft. As the motor strikes the shim it should cause it to vibrate at a frequency that the tuner would turn into a note reading. If you were to look up the frequency for that particular note, some quick math would give you a reasonably accurate RPM. 

The problem with this method is that most industrial machinery is not located in a concert hall. The ambient noises in a typical factory reduce even the most careful measurement right back into guesswork. So, if we generate an electrical signal instead of relying on sound, we can isolate and refine our measurement into something useful, and gain a tolerance of around 5 percent.

You'll need: 
A small permanent magnet
Old headphones
Enameled magnet wire (pretty fine; mine feels around 0.010")
A digital guitar tuner with an input jack (you can use a good one - we won't be modifying it in any way)
A 1/4"  male to 3.5mm female plug adapter 
The patience to splice very thin, very fiddly wire

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Step 1: The Sensor

The sensor is as cheap and easy as it gets. I picked up a package of 1/2" ceramic magnets from Radio Shack for around three bucks. (I buy these five-packs every time I need one magnet, and then the rest vanish. Apparently, they're biodegradable, or I have mice that are far more sophisticated than I would expect.) By wrapping some fine enamel-coated wire around the magnet (see photo) we have a magnetic pickup, like in an electric guitar. It's tough to see in the picture, but there's probably fifteen or twenty wraps of wire on there. I didn't worry about north or south poles, and I don't think that would really have an impact on function.

Step 2: Add Wiring

Lop the connector off of a set of old headphones, and splice on your magnetic pickup. I used a meter and Identified which point on the plug went to which wire - most headphones these days are stereo, with a discrete left, right, and common return -  and wired the pickup onto the return, which should be the top (furthest from the wire) conductor on your plug, and either of the other two wires. Chop off the remaining wire and heatshrink everything together. You should now have something that resembles the picture.

Step 3: That's It. Go Measure Something.

So, now you have a magnet on a string. That doesn't help much, does it? How do you use this thing?

An object made of ferrous steel moving through the field generated by the magnet will affect the field. This, in turn, will induce an electrical current through our coil of wire. If the steel object were the head of a bolt attached to the spindle on a drill, every time the bolt head passed by, the guitar tuner would think it was a guitar string vibrating. By comparing a musical note reading from our tuner to a chart like this one,


we see that low C is 16.35 Hz, or 16.35 oscillations per second. That, times sixty seconds in a minute, gives us 981 RPM. If you had multiple cams (protrusions) on your shaft, you would divide your measurement by the number of cams. In the picture, you see that I used a paddle bit in my drill and read an A sharp at half trigger, and since it's a hand drill (max 1500RPM) it ought to be pretty slow, and this guitar tuner that I've got is only good for the first octave anyway.

A# = 29.14Hz x 60sec=1748 divided by 2 (since there's two impulses per rotation, one per side of my paddle bit) =874 rpm. Give or take. 

During testing, I did discover that the non-grounded, non-shielded drill gave quite a bit of interference, so I put a three-foot headphone extender between the tuner and the sensor. That ought to allow you to hang this thing wherever you want, since the sensor itself is so small.

Also, check your tuner - some are good for multiple octaves.

Any questions, give me shout.

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    4 Discussions


    7 years ago on Step 3

    Very helpful and the links were the best. I can't wait to build my own. Can't wait and see what you got coming next.
    Thanks for DIY project. Would have never thought to make it out of a tuner. Till next time.


    7 years ago on Introduction

    In #3 it should read
    Many CNC and other machine tools have a RPM output FROM THE MOTOR (built in opto or magnetic sensor). Connect that to a tach or frequency meter like in #2.


    7 years ago on Introduction

    A black magic marker and a photo tach (eBay $13) would be easier and SAFER. It isn't safe for you or the machine to hold something against a rotation shaft. If its circular as most rotating shafs usually are then unlesaa they have a grove or protrusion this wont work well.

    My method: #1 Best one
    Use a black Permanent marker to paint about 1/2 to 1/4 for 1 section per rotation and hold up a laser or photo tach. Or attach an opto sensor in series with a 10K resistor powered by 5v for input to any tach. This way you know the speed without affecting it. For a CNC head that moves just attach the opto sensor and run the wires on the machine axis with the other wiring.

    Many digital meters also have a frequency measurement setting. Use the opto sensor like in the last paragraph but measuring the frequency output and multiply the result x60.

    Many CNC and other machine tools have a RPM output (built in opto or magnetic sensor). Connect that to a tach or frequency meter like in #2.