Reading PC Fan RPM with an Arduino
Reading the RPM of a common 3 wire PC fan is quite simple. First you must connect your fan to the Arduino. In this instance we will be powering the PC fan with an external 12v source. The schematic in the fritzing file and the image gallery will fully explain how to hook up the fan. You must also connect a 10k pull up resistor from the fans signal line to the arduinos 5v pin.
The code for this project was written by Crenn from http://thebestcasescenario.com