Arduino Tachometer

Data & Observations
           Here is the demonstration video of how the system works given input and resulting output. A fan is placed inbetween the IR break-beam link and things go from there. go watch:

           The two fans operate at around 3000 RPM and 2600 RPM which was reflected with an error of about +/-100 RPM. I attribute that mostly to round-off errors in the calculation process. Either way hopefully you are convinced that the system works as designed.