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Tools for measuring rpm (revolutions per minute) of a rotating device


Easiest Solutions

Bicycle Speedometer

You can also use a digital bicycle speedometer and do the math for the diameter of your wheel.

Mark Hayton

When magnets are involved

When you have magnets involved in your rotating device, a magnet-based tachometer will most likely not work because the magnets in your system will drown out the magnet of the tachometer (e.g. in a bicycle speedometer).

Optical Tachometer kits

There are Optical Tachometer kits for $28, you might be able to get them cheaper if you look around, or there may be a circuit diagram that it posted to build it from scratch.

Here is the link to a kit..

Also I bought an optical tachometer on eBay for $50, so that is a source as well.

Mark Hayton

Laser Tachometer

I recently purchased a Laser Tach for $49 from Marlin P Jones. I like it very , Tel 541-848-8236 Gerhardt


Hand held photo reflective tachometer uses LASER beam. 5 digit LCD display measures RPM from 2.5-99,999. Resolution: .1RPM /2.5-999, 1RPM/above 1000. ... Requires reflective area on a non reflective surface. $49.00


Hand held photo reflective tachometer. 5 digit LCD display measures RPM from 5-99,999 with auto range select. Resolution: .1RPM <1000, 1RPM/above 1000... Requires reflective area on a non reflective surface. $39.00

Multi-meter with Frequency Option

by Joe Nov. 8, 2004

Just get a multi-meter that has a frequency counter option. ($60 max)

You can place a small separate coil (hooked to the meter) next your spinning rotors magnets (I prefer this method), or measure the frequency from your existing pulse coil (no added drag to motor at all, but sometimes you get strange readings)

If you read 100hz, and have a 4 magnet wheel, then you divide the 100hz by # of magnets (4) = 25hz (cycles/rotations per second)

Then multiply the 25hz x 60 (seconds) to get RPM reading.

Again, take the frequency reading (hz), divide by the number of magnets on your rotor, and multiply that number by 60 for RPM.

So, with a 100hz reading, using a 4 magnet rotor, you have a speed of 1500rpm.

Makeshift Oscilloscope

Here's the cheapest solution I know of: In the files section you'll find a file "scope from your sound"

Install it, and then take a small coil of fine gauge wire, and wire it up to a 2.5mm male headphone jack plug, and plug that into the microphone input on your sound card of your computer. Then position the coil near one of your magnets.

As the magnet passes by the coil it will induce a small current into the coil that will then register on your "sound card oscilloscope". The scope will allow you to read the frequency of the pulses. Divide your frequency by however many number of magnets, and you'll have your RPMs. The coil should present a very tiny amount of drag on the motor, I suspect so small it would be difficult to measure.

Alternately, you can run a few extra turns around your drive coil, this will also allow you to not only see the frequency of the pulses, but also what the pulse generated by the coil looks like (although you won't be able to tell the voltage/amperage).


Dave Narby

Mechanical Tachometer

There are also cheap mechanical tachometers you can pick up on [ eBay].

"digital tachometer kit"
"stroboscope kit"
"optical tachometer kit"

Mark Hayton

Model Airplane Industry & NIST

The RC model airplane industry has some tachometers that might work from about $20 and up. Check out: - should take you to the tachometer page. On my browser, the pictures don't work for some reason. However, you can go to their home page and get images of their paper catalog.

Also: - search for tachometer in the search box.

Here are a couple of particular manufacturers' models: - tachometer - tachometer with limited volt meter.

Note, this volt meter is designed to measure the status of rechargeable batteries. It will put a load of 250mA to 400mA on the battery being measured. This makes the battery reading more accurate but may be a problem in other circumstances. Keep in mind that the volt meter is not high accuracy and only reads to the 10th of a volt.

These units are designed to read rpm from the ambient light going through a propeller on a model plane. They are generally selectable for 2, 3, and sometimes 4 blade propellers. Once you select the propeller type, the rpm is automatically displayed, and you don't have to do any math. They don't require a separate light source. Just medium to strong background light. The one I have will not work with fluorescent lights, as the light itself is flickering at what appears to be 3600 rpm to the instrument.

To use one with a Bedini type wheel, simply arrange to have 2 symmetrical protrusions on the interior or exterior of the wheel or on the axle if you have a turning axle. Set the tachometer for a two blade propeller and position it so the protrusions interrupt the light to the photocell.

If you want something more versatile, precise, traceable to NIST standards, and expensive, try - search for tachometer in the search box. - This link may or may not work. This is Grainger product number 3T179 and is the Monarch Pocket Tach 100. It can be used in contact or non contact mode. It has its own light source and uses reflective tape on the target. It has many functions and features and costs $ 195. The only disadvantage I know of on this item is that you have to hold the trigger button to take a measurement, you cannot let it just sit there.

Just a side note. Check out Circuit Cellar magazine. It has lots of information about using data collection and microcontrollers as well as cool ads. They also have lots of references on their website. Those who want to trap their data might consider a data logger device. Try this link: .

Hope this helps.


Ron Frazier

Related Links

"digital tachometer kit"
"stroboscope kit"
"optical tachometer kit"

See also

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