Measuring RPMs with a Digital Multimeter

[LittleBrother]

I have a Digital Multimeter (DMM) which also measures frequency. Can this be used somehow to show RPM's of a DC motor while it is running at various stable speeds? Or is there an inexpensive device which can be used to show RPM's?

If I have to build a device, my thoughts are to mount a sort of flywheel on the shaft of the motor with a magnet attached to the flywheel. The magnet would pass by a Hall-effect sensor. Then I could feed the output from the sensor to the DMM. Would this work?

An alternative would be to use a flywheel with a slot cut into it, along with a light source aimed through the slot, and a photoresistor on the other side, then feed the output to the DMM.

Thoughts anyone?

 

[enigma]

I think your alternative idea would be the best. I've got a feeling that the photo-sensors would be much less noisy than a Hall probe.

 

[Integral]

You are making what is called an encoder (consider that a key word for a web search) A common low precision form of encoder, a vane encoder, is similar to what you describe, frequently the wheel will have multiple slots.