[dtengineering]

A more basic approach might be to use a reflective object sensor, such as a QRD1114 (or OPB706A, or any of a wide range of IR reflective sensors) and a bit of reflective tape on your wheel. If your wheel is already reflective, then use non-reflective tape... anything so you have part of the wheel reflective (shiny, white) and part of it non-reflective (dull, black). The non-reflective part could also consist of a hole in the wheel... anything that doesn't reflect IR light back into the sensor. You have to add a couple resistors to the QRD1114, but you'll find plenty of tutorials on how to do it.

This way you'll get one pulse per revolution... just use the RoboRio to time the interval between pulses and you're set. You can achieve the same thing by looking at the shaft... paint half of the shaft flat black, and leave the other half shiny.

Do note that the effective range of the QRD1114 is about 1mm. And that it has 1114 in it's name. If it's 1114, that means it's got to be good, right? :-)

Jason