Suggestions for tach generator or dynamo?

Any suggestions: Where can I find a simple speed feedback to my Uno from two 10" dia wheels. I remember as a kid I had a bicycle which had a small tach generator dynamo driven off the back wheel to power the lights. Not sure if it was dc or ac. Thanks for any suggestions.

I think I found it myself:

You could actually simply fit small magnets (1 or more) onto your wheel(s) and by using a reed switch time the speed of rotation. Intergrate that over 1 minute and there you have revs/min. Much more reliable than a tacho with only one moving part that is good for at least 10e6 pulses.