Sometimes you just have to know how fast a wheel or shaft or motor is turning.
The measuring device for rotational speed is a tachometer. But they are expensive and not easy to find.
Its cheap and easy to make one using a bicycle speedometer (cyclocomputer). In fact, the only thing you need is a functional cyclocomputer that reads speed in miles per hour. You won't damage it, so you can even 'borrow' one from your bike, or add it to your bike once your done!

You will only need 1 thing:
A cyclocomputer that reads in MPH and that lets you enter a wheel size in millimeters. Almost all of them do.

You will have to mount a magnet to your spinning wheel, engine shaft or what not.
And you will have to mount the speed sensor near the path of the spinning magnet.

Thats it!

Step 1: Theory and numbers - skip if you don't care!

A cyclocomputer calculates the speed your bicycle is traveling by sensing how fast the wheels of the bike are spinning. It senses wheel speed through a magnetic switch that is mounted on the bicycle frame or fork, near the path of a spinning wheel. There is a magnet attached to a spoke of the wheel and when it goes by the magnetic switch, the switch closes for a moment, which is recorded by the cyclocomputer. By timing how much time goes by between switch closings, it can compute how fast the wheel is spinning.

When you first install a cyclocomputer on a bicycle, you have to enter the circumference of the wheel in millimeters. With that information, it can compute how far and how fast you've gone with every turn of the wheel.
In this instructable, we are going to enter a special number into the cyclocomputer for the wheel circumference that will trick it to report speed in rpm.

If the cyclocomputer is reporting speed in mph, enter 268 mm for the wheel circumference.
If the cyclocomputer is reporting speed in kph, enter 167 mm for the wheel circumference.

Here is the calculation...
1 mph = 1.61 kph.
1.61 kph / 60 minutes in an hour = .026833 kilometers per minute
.026833 kpm* 1,000,000 millimeters per kilometer = 26,833 millimeters per minute
26,833 mmpm / 100 (scale factor) = 268 mm
268 / 1.61 mph-kph conversion = 167 mm
Note: some cyclocomputers might not be able to accept a number below 200, so use the 268 mm number, reporting in mph might be better.

If you ride, you may know that some cyclocomputers also record cadence, which is how fast you are pedaling - and that number is in RPM! So the cyclocomputer already is a tachometer! But they can only read to a maximum of 199 RPM, which is way faster than a person can pedal anyway. I guess if you needed to measure a really slow speed it could work, but this instructable lets you measure a much wider range of speeds.
<p>I tried this but when i put the sensor near the magnet, it read a max of 9.8 mph. even though i kept revving it once it had reached 980rpm.</p><p>thanks. </p>
<p>but my speedometer uses cm instead of mm...and i cant input decimal numbers</p>
<p>I would assume that requiring 4 digits implies metric input, and that 268 would mean 268.0, input as 2680....</p><p>Also, your provided information is confusing. I suspect you might have to mentally multiply or divide the displayed number by 10 (or 100) for RPM. <br><br>Remember, cyclometers actually measure nothing but how many times a minute a magnet passes near a sensor (RPM). Then, the internal electronics convert that number (based on wheel diameter) into a speed. By &quot;fooling&quot; the electronics with the calibration numbers provided by Josie, the output is converted back to a number which indicates RPM... but we may need to mentally multiply the number by 10 or 100 or 1000 for the proper number. The computer does not know it's being fooled. It still thinks it's doing its job of reporting MPH. Just ignore or tape over the letters that say MPH, if that's the case.<br><br></p>
<p>I also entered 2680 and 1670. Same results- speed but not RPMs.</p>
<p>I purchased a bicycle speedometer, and following your instructions, hooked it up to my water turbine that I run my water powered sawmill with. It works fine as a speedometer. However, the instructions that came with the cyclocomputer require that you enter a 4 digit number as the value for the mm of one revolution of the bicycle tire. Your instructions require entering '268' or '167' , both 3 digit numbers. I tried entering 0268 and 0167. I do get a variable readout, apparently of MPH, and this readout increases as I increase the speed of my turbine, but MPH is not what I'm looking for. the whole idea is to have a tachometer. The diameter of the wheel that I have hooked up the magnet to is 10&quot;. Any ideas?</p><p>Thanks</p><p>Steve </p>
Thank you Josie, <br> <br>I considered a wireless bicycle tach but I believe they dont have the range I need.
I am going to use a Bicycle Tach for a rotor tach on my Gyrocopter. This means I have to extend the wire from the sensor to the display unit. It also has to display a range from 0 to 400 RPM. Upon cutting the wire to extend, I notice it has two very fine wires within it. I think this is true but I cannot even see a wire. How can I extend this and will your calculations work for my applications? Thank you
Unfortunately that fine wire is a real pain to work with. To connect to it use a cigarette lighter to burn off the lacquer that covers the strands and then scrape the wire clean with a knife. Then solder to it. I bet you could extend the wire some distance without problems. You could always pick up a wireless cyclocomputer if that would be easier, but more $$$. Good luck.
I have a question , I have this attached to my VAWT and have a balance issue, does it matter where or how close to center of the disk&nbsp; you place the magnet or does it not matter since it counts revolutions.&nbsp; Will mounting the magnet to the shaft still have the same result as mounted on the outer edge of the disk?<br /> Hope your still around. Great Idea!! <br />
It makes no difference where the magnet is placed. <br /> If you are having a balance issue, you could&nbsp; attach a non-magnetic weight to the disk opposite the magnet.&nbsp; This should even things out.<br /> Glad you are using the tachometer.<br /> BTW what is a VAWT?<br />
Very nice 'ible. Excellent way to re-purpose a porduct for something else. <br /> <br />BTW - VAWT = Vertical Axis Wind Turbine. (has the advantage of being able to turn regarless of the wind direction, compared to a standard wind turbine which has to face into the wind before it will start to turn)
Awesome, thanks Josie! I was just finishing a cup of coffee on my way out to buy a $60 tach. I put engines on bicycles and need one to do gear ratios. Notice in pic on &quot;dashboard&quot; $40 tach, snork. Again, very cool and thanks for sharing!
Hello Josie.<br /> Here are 2 pic's of the two I am using to test my turbines.<br /> &nbsp;The first one is the transmitter type good up to 2 feet. sorry for the fuzz.<br /> I recommend the wired type for permanent attachments, second pic. I have that attached to the turbine I have in the roof, spliced with CAD to run it to my work shop 30' away. Unfortunatly it only works when the wind is blowing.<br /> <br /> The first Pic is atached to my little turbine the 2nd is attached to my large turbine on the roof of my truck at ocean shores. The weather station for wind speed. <br /> I think I&nbsp;have the number set 2680mm.<br /> <br /> Thanks Josie, this is a lot of help building my turbines.<br />
Hi,<br /> This is great, simple, cheap, easy, and serves my purpose.<br /> <br /> I wanted to make a simple tachometer myself for a windmill project (slow revolutions), and surfed around a bit .. found systems based on lasers, mechanical counters, converting old cd players, building a circuits , etc .. all of this is rather complicated&nbsp; when you're not specialised&nbsp; ..<br /> <br /> But then I saw your pages .. found an old cyclometer, I got with a box of Kellog's Cornflakes .. stuff for kids .. changed the value of the wheel .. and VOILA ! <br /> <br /> Many many thanks, for this easy and very logical convertion, because I was not bright enough to do the thinking process (da numbers) .. and you did it, for us all to see!&nbsp; :-)<br /> <br /> Cheers<br />
excellent idea. how are you using it? have pics of actual usage? thanks.
This worked perfectly for revealing the rpm of an electric motor. My computer only uses cm, so I had to round up to 27cm, but it worked just the same. Great, great tip! I'm telling *everyone*!!!
Just finished this conversion...thanks for figuring out the hard part! Having just switched from a DF bicycle to a recumbent, I was interested in cadence but not for $45. I knew there must be a way to convert a speedometer but didn't want to re-invent the wheel. Found this site and boy am I glad. Now, when someone asks why I have 2 speedometers, I'll just tell them that my bike is so fast that only 1 speedometer wont measure all of it! Thanks again!
Sounds hard. I don't even know what <em>tachometer</em> means, but it looks like a hard thing to do, and hard things to do are most of the time really cool. :D<br/>
josie hope I am not stepping on any toes by answering. Gorillazmiko, tachometer is a instrument that displays RPM Revolutions Per Minute, most car's and motorcycles have one. This is very easy thing to do if you live here in the states you can buy a cheap one at wal-mart for under $15. Follow the directions when it comes to wheel size use the numbers given here for what ever application you are using it for. Motor's and such will use 268mm and for like me, a bicycle is 2680mm. Both the sensor and computer, display or in this case a tach are installed using zip ties which are should be provided. All Credit to josie.
haha <em><strong>taco</strong></em>meter<br/>
Really a great way to accurately measure rotational speed. Not sure how accurate, nor the settings to use it as a tachometer. (select radius of the wheel to equal radius of the rotational object?). An interesting way to measure , say , the actual speed of a turntable platter accurately (as long as accurate to .1 +/_ .05 units...)...
I am so doing this. I am a avid cyclist and I have been wanting to check my RPM which is cadence. I thought of doing this and I new all I had to do was figure out the number to use in the wheel size part of the computer, but I just could not think of how to go about figuring out the math.. THANK YOU so much I love it and love this site you have made me very happy. I am so doing this tonight when I get home from work. Hope you do not mind I will be passing this around to my riding friends. I have a question I was going to mount the magnet to one of my chain rings does it matter where or how close to center do you need to be or does it not matter since it counts revolutions.
You definitely can use it to measure cadence. Most cyclocomputers that have the cadence function put the magnet on the left crankarm and the sensor on the left chainstay. One suggestion: I would enter a wheel size of 2680mm instead of 268mm. That way 60 rpm will show up as 06o and not 00little6. This thing was meant for showing speeds on motors and the like, not for cadence, but now that you gave me the idea, I might hook up an old cyclcocomputer to my bike and try it!
Thanks Josie for the the wheel size increase. I know there is so cadence computers for cycling and some of them actually calculate your power output some of those get into the thousands of dollars. I let my friend borrow my spare comp. but I should have it sometime this week I was going to mess with mine at home but it has a lot of info logged on it so I am going to impatiently wait. I will post up when I get it done and let you know. Thanks again MotoGP starts up this weekend go Valentino Rossi 46!!!
I featured this because of it's novel qualities - but, I really think better pictures would be nice (such as an actual picture of the sensor mounted on the motor).
I wanted to show a pic of the sensor mounting. But I have it hooked up to a small gasoline engine, and the sensor is under the cover where you can't see it. The magnet is the one on the engine flywheel, so it doesn't even look like a magnet! Maybe I'll set something else up to get a decent pic though.

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