dave-in-nj:
...but you are probably only a few minutes away from success.
You're right, sir dave-in-nj! however, my error is not because of the material.. it's the wiring 
Anyhow, thank you for not letting me give up!
Changed my wiring from this,
R1=220, R2=220
to this,
(for Arduino Tachometer - Software | PyroElectro - News, Projects & Tutorials)
(for http://www.instructables.com/id/Arduino-Based-Optical-Tachometer/?ALLSTEPS)
R1=100K, R2=220