How to convert mechanical RPM to a digital signal?


Help! We’re working on a project here to use a bike mounted on a stationary bike trainer and, based on how fast the rider is pedaling, can be used to control the speed of a pump within a machine.

The machine uses a rheostat (like a light dimmer) to interface with its computer and thereby control the pump speed. We’d like to somehow use the bike setup and use the pedaling cadence to replace the rheostat. So for example, 100 rpm on the bike will equate to the max on the rheostat, and 0 rpm to 0.

Can anybody suggest some sort of device or solution to measure something that’s spinning (rpm) and convert that to an electronic signal? THANKS!

An electric eye of some sort would do the trick. The sensor could be magnetic or light sensitive. The sensor is stationary and the mating piece is on the wheel. Each time the wheel turns, you get a signal when the sensor sees reflected light or is energized by the magnetic field. I’m sure there are other ways to do it, too. The computer puts a time stamp on each record. Then you can do anything you want with the data.

Buy a bicycle speedometer/odometer for about $20. It’s basically an interval (wheel revolution) to rate (speed) converter. You can take the signal from the display and interface it to whatever you want.


My mistake, thought this was a serious endeavor…

Once upon a time there were bikes with a built in front wheel hub generator that operated the front and rear light…the faster we rode the brighter the light…can you use one of those wheels? There was also one type that mounted on the front support and rubbed on the tire.

Thanks for all the responses. I was thinking one of two ways:

  1. Use a cycling computer with a cadence sensor, which would tell me rpm on the digital display. The question is how would I be able to connect that digital output to interface with the pump controller?

  2. Use a wheel mounted dynamo powered bike light. The more/faster you pedal, the brighter the light becomes (to a point). So here we have an electricity signal that is directly proportional to cadence. Is there an amplifier device that can take this signal and use it to vary/control of much electricity from a 220V 30A outlet to power a DC motor pump?

Knowing nothing about this…I am sure there is a step-up transformer available that would work.

Why do you need to measure cadence (rpm at the bottom bracket)? Shouldn’t you be measuring revolutions at the rear hub/wheel? If your bike has more than one working gear cadence wont tell you anything, assuming your machine is linked to the trainer resistance unit.


Perhaps you mean the Sturmey-Archer Dyno-Hub?  The question would be is there a linear relationship between RPMs and brightness.  I guess it would be the current that increased with speed, not voltage?  One of many areas where my knowledge could justifiably be called slim.

Good point. Either cadence or rpm of the wheel will do. How can either one of these inputs control a 220V DC motor pump?

@Wes Sprinkle:

The OP specifically said "based on how fast the rider is pedaling".  It's a bike trainer, so there's no need to assueme that there even IS a rear wheel, much less that the rider would be changing gears.


This sounds like a job for the Arduino.  Google it.

Bikes mounted on trainers have wheels, stationary bikes don’t. The language in the original post is a bit vague, but it sounds like a bike with wheels to me.

Sounds like a bike without wheels to me, or at least the OP doesn’t care about the wheel’s RPMs.

And yes, there are stationary bikes that DO have wheels. The language in the original post is quite precise: “based on how fast the rider is pedaling”. Thank you for playing our game.

Via an Arduino. Google it.

What exactly are you trying to do? Why use a bicycle to control the speed of a pump? Why not just use the bicycle to power the pump and be done with it? Is your real name Rube Goldberg?

What does a stationary bike with wheels look like?

I’ve been wondering the same thing as “ZombieWoof”

Are you making a music video, a la OK Go (

Tried to that already. It would require a 53 on the chainring, 11 on the cog, another 36 chainring mounted on the other side of the rear hub, and finally to another 11 tooth cog. Basically, the calculation suggests that in order to achieve the required rpm of the pump, the cyclist would have to put out about 1,000 watts of power. Pro cyclists average 300-500 watts on the course with short peaks maxing up to 900-1,000 watts. This obviously won’t work with most people.

My stationary bike had a wheel. A spoked wheel in the front fork with a simple adjustable resistance wheel in contact with the wheel’s hard rubber “tire”.

However, a bicycle computer (odometer, speedometer, etc. tha computes just about anything you’d like) works with a magnet and a sensor, the magnet attached to the wheel and the sensor to the fork or framework. Today’s units are infinitely adjustable to base the computations on the actual rolling circumference. Any bike, stationary or not, that has any form of rolling disc or wheel can have one mounted on it. They come with them, but an additional one can be added to meet the OP’s goals.

However I liked the idea of using a bike headlight generator. That’ll provide a variable DC output that can then be used to control the 220VAC output. I have to admit that finding an amp of that type may take some research by the OP, perhaps with assistance from a place like Radio Shack.

The output of the bike light generator may be on the generator or its box, the supply line voltage is known, and I’m sure the OP can identify on the pump label’s input voltage requirement, so perhaps something exists to meet the need. Once these parameters are known, a search for power supplies on the internet may yield some results.

One example looks like a stationary bike with a flywheel. The rest I will leave to your research. I am right you are wrong.

Gotdam, this is butt simple. ARe you trying to control the pump, or trying to power it?

If you’re trying to power it, you’ve already stipulated you have a 220V power source.

If you’re trying to control, hire a drummer and buy him a MIDI drum pad.

It’s not really clear what the object of the exercise is.