Tachometer Made From a Bicycle Speedometer (cyclocomputer)

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Introduction: Tachometer Made From a Bicycle Speedometer (cyclocomputer)

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.

Step 2: Set This Thing Up!

Program the cyclocomputer by entering 268mm for the wheel circumference, and make sure that it reads speed in miles per hour.

Mount your magnet on the spinning part.
Mount your speed sensor near its spinning path - no more than 1/2 inch away.

Step 3: Fire It Up!

As soon as your device starts spinning, the cyclocomputer should read the speed.
The big numbers are its speed in hundred of rpm. In the picture, it is spinning at 2810 RPM.

The best part about using a cyclocomputer is that it automatically records how much time the device has been spinning. In the picture, it has been running for 0 hours, 13 minutes, 21 seconds. This could be useful if you wanted to (for example) change the oil on an engine every 100 hours of operation.
The cyclocomputer also records maximum and average speeds, which could be useful in some situations.



Troubleshooting:
If no speed shows up, try moving the sensor closer to the magnet, and make sure that the cyclocomputer is set to display speed. If it still reads nothing, move a magnet back and forth past the sensor by hand. Numbers should appear. If they don't, the wire could be broken. They are pretty flimsy.

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    26 Comments

    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.

    thanks.

    but my speedometer uses cm instead of mm...and i cant input decimal numbers

    I would assume that requiring 4 digits implies metric input, and that 268 would mean 268.0, input as 2680....

    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.

    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 "fooling" 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.

    I also entered 2680 and 1670. Same results- speed but not RPMs.

    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". Any ideas?

    Thanks

    Steve

    Thank you Josie,

    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  you place the magnet or does it not matter since it counts revolutions.  Will mounting the magnet to the shaft still have the same result as mounted on the outer edge of the disk?
    Hope your still around. Great Idea!!

    It makes no difference where the magnet is placed.
    If you are having a balance issue, you could  attach a non-magnetic weight to the disk opposite the magnet.  This should even things out.
    Glad you are using the tachometer.
    BTW what is a VAWT?