The main reason I built this is because someone I knew wanted a stand alone device that is capable of measuring/test CPU Cooler Fans independent of the computer. And here it is, a PIC16F627 based CPU Cooler Fan Speed Tester. The speed is displayed on 4 units of 7 segment display as shown.
The TMR0 and TMR1 is utilized together to capture the pulses and the 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 3600 RPM.
Since there is quite a few 7 segments displays, there will be one hell of wiring to do if all segments were to be controlled simultaneously. Therefore to minimize the amount of wiring and current consumption, a switching method is used. By using individual transistor to switch on/off each segment, the PIC will display one number at the time on the seven segment displays. For this project, it will display the numbers from right to left. Logically speaking, it is a matter of the following. Let us say, the RPM measured is 4189 RPM, then the following happens,
Display 9 on the 1st segment from the right, then delay 20ms.
Display 8 on the 2st segment from the right, then delay 20ms.
Display 1 on the 3st segment from the right, then delay 20ms.
Display 4 on the 4st segment from the right, then delay 20ms.
This method will give the viewer a "perception" that all 4 of the 7 segment displays are simultaneously display the RPM.
The emitter of the transistors appear to be alittle warm during the operation of the device. To cool it alittle, it is recommended that a heatsink to be used to reduce the stressload on the transistors. Otherwise, the heatsink is not really needed.
Schematics : FanTesterschem.jpg
