# quick, messy, & cheap spindle RPM meter / tachometer



## spaceman_spiff (Nov 12, 2014)

Ingredients:

frequency meter (DMM/multimeter, oscilloscope, guitar tuner, computer/phone with microphone and audio software)
resistor (~100k)
capacitor (~0.1uf)
battery (9V)
various wires/probe leads/alligator clips
piece of scotch tape 
way to multiply two numbers

How it works:

You are creating an electrical square wave pulse train from the rotation of whatever object you want to measure.
Then you measure the frequency of that pulse train with your frequency meter.
The object must be metal. (Unless you instead make an AUDIBLE square wave like cards in bicycle spokes! And then use guitar tuner or phone/computer with audio analyzer)
You connect things so that you touch one probe of the meter to the machine body (lathe ways, mill bed, etc..).
The other probe gets held gently against the spinning spindle of the machine.
And you place a piece of scotch tape around 1/2 the circumference of the spindle. So the meter will measure battery voltage half the time, and nothing the other half. There is your square wave.


ACTUALLY. That wont work. Because the meter probably wont recognize battery voltage and then "nothing" (60hz noise) as a square wave.

So you need to "pull down" the meter probe to battery negative (assuming you connected positive to machine).

Thats what the resistor is for. The resistor goes across the meter probes.

OOPS..still doesnt work. Too much noise. Thats what the capacitor is for. Place it across the meter probes too. Now we've filtered out some high frequency crap.

Ta-da! My Atlas 10F was spinning at 9.8Hz. Multiple by 60 and you have rpm. 588 rpm! Wow alot slower than I thought.


REALITY CHECK:

Different meters have different requirements for frequency measurement signals. For instance some may not measure frequencies as low as 10hz, or some may need the voltage to be above 10V or below 5V or who knows what. Gotta check your manual and adjust this circuit as needed.

Lotsa wires all over and holding a probe against a spinning lathe chuck is dangerous. So dont attempt any of this unless you want to play the "stuff flying around the machine shop" game.

You may need a different resistor and/or cap value. For me 100k and 0.1uF worked great with a steady frequency reading. Careful you dont filter your signal right off to zero! 

Dont event THINK about using mains voltage instead of a battery!

Dont leave the battery connected. Dont use too small a resistor or you'll kill your battery and maybe the resistor too. 

Now you can check all your pulley speeds once and know exactly what they are! Im not sure where some of my pulleys came from on the countershaft and motor, so its nice to verify everything.

If you dont have a meter which reads frequency, or doesnt read frequency this low, you have some options:

-Instead of putting one long strip of tape around 50% of the spindle, make a bunch of little squares, so now you have a much higher frequency, which maybe your meter can read.

-If your meter doesnt measure frequency, perhaps you could read the pulses as a voltage and if you know your meters averaging/response you could interpret the voltage that way.

-You could make an integrator and average the pulses into a steady voltage which you read with the meter. The voltage would be proportional to the frequency but you would need to know the duty cycle I think. That or carefully make sure the tape is exactly 50% of the spindle circumference.

-You could use an oscilloscope, even an old analog one.

-If you REALLY want to scrounge, you could put a TON of scotch tape "pulses" on the spindle, as many as you can fit, which should put the frequency up into the audio range, and then you could try measuring it using a guitar tuner, or perhaps a computer with software that lets you measure frequency or look at the waveform and figure it out. For machine that has only fixed pulley speeds, you only need to do this once, so even if its a mess and pain it works out.  But for something that has an infinitely variable speed, you may want to make a tidier setup that uses a meter you can always see.


HAVE FUN! A MILLION WAYS TO SKIN THIS CAT!


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## spaceman_spiff (Nov 12, 2014)

So I just set my Atlas 10F to the highest speed pulley setup and measured the rpm, expecting it to be around 2000rpm, which is what I had been assuming.

860rpm!!! You know what they say about assumptions! Ive only had the lathe for about a month so thats my excuse. 

I couldnt believe it so I looked at the motor RPM and measured the pulleys...and yep thats right where it would end up.

No wonder I've been having so much trouble cutting small diameters. 

I'm going to need to basically toss the entire countershaft system made of wood-filled tubing and make a sturdy one, and one that doesnt bounce all over the place and require me to put different sized wood blocks under it for every pulley combo so that it doesnt bounce around.

Im excited to try out much faster rpms....


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## SG51Buss (Nov 12, 2014)

A fun project.  I've thought about using a cheap bicycle speedometer, with magnet trigger, and some sort of conversion factor.  But, wimped-out and bought a $13 tachometer.

http://www.hobby-machinist.com/showthread.php/26009-atlas-lathe-tach


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## spaceman_spiff (Nov 12, 2014)

SG51Buss said:


> A fun project.  I've thought about using a cheap bicycle speedometer, with magnet trigger, and some sort of conversion factor.  But, wimped-out and bought a $13 tachometer.
> 
> http://www.hobby-machinist.com/showthread.php/26009-atlas-lathe-tach



If I ever install an infinitely variable speed motor setup imma have to do alot better than the multimeter jury rig lol..$13 sounds nice for a tach. But I think i'd want to permanently mount something in that case since I'd probably be looking at it all the time..have you thought about doing that?


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## Micke S (Nov 12, 2014)

Infrared tachometers made in China is about 10 dollars with shipping, including 9V battery :yikes:. I have this model and it works perfectly fine.

http://www.ebay.com/itm/LCD-Digital...6?pt=LH_DefaultDomain_186&hash=item3f38d19efa


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## SG51Buss (Nov 12, 2014)

Yeah, I've thought about a permanent mount system, have another couple tachs for that project.  Project #83, and slowly gettin' pushed down...


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## Waldo1 (Nov 12, 2014)

Steve, where did you buy that little digital tach? That looks like something I've gotta have.

Thanks!


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## francist (Nov 12, 2014)

spaceman_spiff said:


> If I ever install an infinitely variable speed motor setup imma have to do alot better than the multimeter jury rig lol..$13 sounds nice for a tach. But I think i'd want to permanently mount something in that case since I'd probably be looking at it all the time..have you thought about doing that?



Nice to see somebody else thinks of bicycles too! Back in the day I used to race a bit (very poorly I might add) but I had a little "cyclocomputer" made by CatEye that clamped to the handlebars. Solar powered, very small. One of the functions was cadence which measured the rpm of the crank as you pedaled. If you could find one like that there would be no need for conversion factors. Used a small magnet as a trigger.

Somewhat off-topic but kind of fun anyway, I once wired an optical transistor into a two-dollar pedometer to count how many times a rat walked past a trap during the night. Worked great, and my boss still talks about it...)

-frank


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## spaceman_spiff (Nov 12, 2014)

i really like the bike tach idea...

long battery life because its meant for portable and long term usage, and probably LCD
small
comes with sensor
cheap


and it reads in RPM? thats the kicker...all the ones I have didnt...but if they read in rpm then this is a cinch and imma go buy one


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## SG51Buss (Nov 12, 2014)

Waldo1 said:


> Steve, where did you buy that little digital tach? That looks like something I've gotta have.
> 
> Thanks!



Hey, Waldo!  If you're referencing that other thread, they're all over eBay.  Like Micke S's eBay link, you could just enter eBay and search on 'laser tachometer'.  There's hundreds of 'em.

This one, just like mine, is currently goin' for 99¢.  Better hurry, only 20 hours left to bid...




Here's the instruction manual in pdf:


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## SG51Buss (Nov 12, 2014)

Just for fun, did an eBay search of 'bicycle speedometer'.  OMG, over 4,000 items!
This one (about $3) caught my eye.  Notice the large LCD number segments in the middle of the display, showing '188.8', where the speed would be displayed.  If this thing could actually display as high as "199.9", might could work out as a tach by ignoring the decimal point.  You program into these things the tire diameter or circumference, and choose MPH or KPH.

If you sit down and grind thru the math, realizing that you could attach more than one magnet, you may come up with a usable configuration...


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## spaceman_spiff (Nov 12, 2014)

what about this:

http://www.ebay.com/itm/New-Hour-Me...r-Motorcycle-Boat-Bike-US-STOCK-/331086473062

It has an inductive pickup meant for ignition systems, but I bet you can figure out a way to trick or modify the input to work with magnet and reed relay or hall sensor type setup. Then you would get both an hour meter and tachometer that reads directly in RPM for the $15 range


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## SG51Buss (Nov 13, 2014)

Now you're cookin'.  That version uses a wire wrapped around a plug wire.  I made a permanently mounted timing light for my hotrod using that inductive trick.  I recall that getting 20v spikes was easy that way.  Not sure if you can get that much with a magnet pickup.  Time to experiment.

Tried "digital tachometer motorcycle" and found this thing, $7.55.  It uses the grounding side of an ignition system, and will respond to 12v spikes.

Shame on you.  Now I have to order one...


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## SG51Buss (Nov 13, 2014)

Well, foo.  So I tried just "digital tach" and these panel-mount things pop up.  New item.  Around $8.55.  Triggering by proximity sensor or hall, up to 5v trigger.  This is the way.


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## SG51Buss (Nov 13, 2014)

Well, foo2.  I can't stop.  Tried "tachometer hall proximity sensor", and found the complete kit, with sensor and magnet, around $13.


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## herbet999 (Nov 13, 2014)

spaceman_spiff said:


> ... which should put the frequency up into the audio range, and then you could try measuring it using a guitar tuner, or perhaps a computer with software that lets you measure frequency or look at the waveform and figure it out.



Did this in my college days. Used an oscilloscope to analyze the waveform created by a spinning mirror. All to measure the speed of light... Apparently my college days were pretty boring.


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## spaceman_spiff (Nov 13, 2014)

SG51Buss said:


> Now you're cookin'.  That version uses a wire wrapped around a plug wire.  I made a permanently mounted timing light for my hotrod using that inductive trick.  I recall that getting 20v spikes was easy that way.  Not sure if you can get that much with a magnet pickup.  Time to experiment.
> 
> Tried "digital tachometer motorcycle" and found this thing, $7.55.  It uses the grounding side of an ignition system, and will respond to 12v spikes.
> 
> Shame on you.  Now I have to order one...



SICK. That is perfect looking for an old lathe too....if you get that up and running I betcha there will be a few others doing the same!!


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## spaceman_spiff (Nov 13, 2014)

SG51Buss said:


> Well, foo2.  I can't stop.  Tried "tachometer hall proximity sensor", and found the complete kit, with sensor and magnet, around $13.



(stands up)

(clap clap clap)

well done. Not as cool looking as the gauge one but if if its that cheap and ready to go, its a winner.

now I just need to add a variable speed drive to my lathe to go with my tachometer lol


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## SG51Buss (Nov 15, 2014)

Well, I just couldn't let this go.  Decided to shop around some more, mostly looking for much smaller components.  Found and ordered a digital tach, 2" x 1", hall sensers and magnets.  These should be small enuff to squeeze into a 6" lathe.  Naturally, won't see this stuff for at least a month, so that'll be for a future project thread.


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## Rick Leslie (Nov 15, 2014)

This is a subject I keep coming back to. I have a gaping hole in the headstock of my mill where the factory tach once rested. I want to fill the space with a modern version but I understand little about the differences in the pick-up methods. The magnetic pick-up I get, but how does the Hall Effect work? Does it use gaps or grooves in the pulley surface for reference? 

Magnets on my lathe spindle wouldn't be a problem, but I don't want them on the mill. If the detach, it could cause all kinds of mayhem in the gear box. I much prefer the grooved pulley method. Or am I just completely misunderstanding the concept? Sorry for the side track.


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## SG51Buss (Nov 15, 2014)

Rick, I see you eyeball-to-eyeball on the magnet issue.  With all that swarf and what-not, I'm a bit concerned.  I may look into a different type of sensor system.

There's four types of magnetic sensors.  One uses a coil that produces a tiny current when the magnet swings by.  The second operates a 'reed' switch.  The third uses a 'reluctor wheel', which may be how your millhead was setup,  The other is this 'hall' device.  It's simply a different type of transistor that gates 'on' when in the presence of a strong magnetic field.  It essentially acts like a grounding/shorting switch, like the 'reed'.  The 'hall' devices used in these sensor/tachometer applications are actually tiny IC's that have supporting circuitry to make this thing work cleanly.  If you like tech data, here ya go:

http://www.alldatasheet.com/datasheet-pdf/pdf/55092/ALLEGRO/A3144.html


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## Rick Leslie (Nov 15, 2014)

Thanks for the explanation. Looks like most use magnets in some form or other. I haven't looked closely at the mill, but if it used a reluctor set-up, seems like there should be some tell tale signs on the pulley. 

Thanks again.


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## Steve Peterson (Apr 26, 2017)

I put the same LED tach on my wood lathe with a transformer that gets powered up any time the speed controller is turned on.  Works great, or at least it did for a few months.  Now one of the LED segments has burned out.  5 and 6 look alike.  Also 8 and 9 look alike.  It is a bit annoying, but not so bad that I want to replace it yet.

My only advice is to put in a stronger magnet than the tiny one that came with it.  This allows it to work with a gap of about 1/8".  Also, the magnet works better on one side than the other.  Maybe the hall effect sensor is polarity sensitive.  I made sure to glue it in with the proper orientation.

Steve


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