Overview
The project shows how 3-wire fans are controlled with PWM while simultaneously having RPM feedback.
Details
The fan controller is very sophisticated since it is capable of running a fan with PWM pulses and receives feedback. To keep the RPM steady at the pre-selected RPM value, the circuit will increase or decrease the PWM duty cycle with this feedback. The revolutions per minute come from the translated feedback from the fan and are compared to the desired pre-selected RPM value. During operation, the circuit can perform self-calibration. For the real RPM to be as close as possible to the pre-selected RPM, the duty cycle is changed.
To obtain a sufficient 5V signal to drive the PIC, a 4049 buffer powered with 5V was used. This would result to the occurrence of green channel while the first yellow channel, or input of 4049, shows the feedback. When the RPM pulse is HIGH, a series of PWM pulses appear on the screen and when LOW, the pulses are gone.
With the parallel capacitor, the output of the fan is capable of driving the CMOS gate by just pulling the third pin up using 1K resistor.
