Experimenting with Arduino-powered Tachometers
An Arduino or compatible board can be used for much more than interesting blinking lights, and one example of this has been demonstrated by Harshad Byadgi through his tachometer project.
You can easily measure incoming pulses from a hall effect sensor, which is triggered by a magnet. By fitting a magnet to the rotating object under question, you then have a pulse-per-rotation. Using this signal your Arduino can then - with some basic mathematics - determine the number of revolutions per minute.
This is demonstrated in the following video:
For more information on how to make your own tachometer, visit the detailed project page.
And if you're interested in doing this yourself, PMD Way can help you with the parts, such as: