PC can display CPU fan speed via simple application. Another option is to go to BIOS menu and read fan speed value. But if you want to have stand alone device that measures CPU cooler fan speed so here it is. It is a PIC16F627 based fan tester with four 7 segment LED displays.
CPU fan sends tachometric pulses via separate wire and can be calculated as RPM. PIC captures and calculates the time by using two timers/counters. One captures pulses and another calculates elapsed time. For instance, by capturing the amount of pulses within the period of 1 second would yield the frequency at which the fan spins, e.g. 60Hz. To convert to RPM it is a simple matter of multiplying the frequency in Hz by 60, which in this case yields to 3600 RPM. Schematic is simple – the most components are used for interfacing LED displays. They are controlled via BCD converter and transistor keys. Fan pulse wire connects directly to PIC pin via pull-up resistor as amplitude of fan pulses is same as PIC power supply – 5V.