Below is video tutorial about PWM fan controller with pulse stretching method to get RPM feedback. On this video explained there are three channel on the oscilloscope. The yellow channel is directly connected to gate of the Mosfet. The green channel is the feedback of the fan. Finally, the blue channel. Whenever the blue channel is high, the peak get the feedback of the fan.
We can controll the speed of the fan using the potentiometer. To see how RPM is the fan, we can se through the LCD. For further information, watch the video.