how to get rid of the contaminating PWM signal

[foxjuly]

Hey Guys, I am currently working on a project of using arduino to control the computer fan. I used the Time1 library of Arduino to generate a 25 kHz PWM signal. The fan works pretty well and its speed can be controlled if I adjust the duty cycle of the PWM signal.
Also, I want to measure the rotation signal from the computer fan. However, what I got is a signal contaminated with the PWM signal, So, how can I solve it ? Attached is the circuit map.
Thanks a lot!

[eblc1388]

You cannot get rid of the PWM pulses, unless you stopped the PWM process.

Your question is a common one and Mr. Giorgos Lazaridis had figured out a way to tackle that. Basically he just stretches one PWM pulse to 100% and measure the speed feedback signal. Then the PWM pulses returns to normal. He called it the "Pulse Stretch method". You can learn how he did it at the following link:

PWM 3-Wires Fan Controller with RPM feedback

[foxjuly]

Thanks, I tried to use a capacitor and it works, I am planning to use a small coil in combination with the capacitor to see if it can further remove the 25 kHz PWM Signal.