arduinoTime:
I would like to try sir goodhen's suggestion but I just want to clarify this... is the code the same for using the blade as interrupt (like this)and using a reflective material (like this)?
I guess the second one is what you need.
It could work with the first one, too. It's just a matter of how reflective the surface is. It needs to be so reflective that the bounced light is intense enough to register as logic level. I doubt using the adc is an option-it's too slow for this purpose. Using an opamp (or a transistor) would be pretty simple if the light levels are too low.