Fan Speed Monitor is another simple design based on PIC16F876A microcontroller. Two similar posts about ways of controlling and monitoring fan speed were recently published. PWM Fan Controller and 2 Channel PC Fan Controller. Current article however implements a different approach to speed monitoring. Fan rotation measurement is done by counting the number of times the fan blade passes a photo sensor illuminated by LED. Photoresistor is located behind the fan as can be seen in the following figure. Number of fan blades should be taken into account in order to produce a correct RPM reading. Arctic Cooling 120mm 7-blade fan was used to test the design. The number of fan revolutions is shown on 4-Digit LED Display. A simple multiplexing routing was applied to drive the 7-Segment Display. See attached assembly code for more detailed explanation. Design schematic is available at the bottom of the page. As a possible improvement alarm can be added in case of fan malfunction by connecting a buzzer to PIC I/O.
