Hi everyone
I "recently" got a new laptop and its fans are very noisy at idle. The two fans have 4 wires each and are controlled by a fan controller built into the laptop's motherboard. The 4 wires are 5V, GND, PWM from fan controller to fan, SPEED from fan to fan controller.
PWM to fan: 24 KHz, 3.3V pk-pk square wave. Fan speed is proportional to duty cycle. Min duty cycle: 25%, max duty cycle: 100%
SPEED from fan: 5V pk-pk, 50% duty cycle, square wave. 2Hz correspond to 1RPM
Testing has shown that the SPEED wire from the fans is not taken into consideration by the fan controller, which only adjusts fan speed according to CPU and GPU temperatures, and so as to simplify the implementation can be ignored. Moreover, the fans can also function with an input PWM cycle of as low as 15% which makes them very silent.
I would like to create a circuit using an attiny85 (i have very little room to work with), that would turn the fans off or down to their minimum 15% duty cycle when the laptop is idling or doing very little workload yet it would allow the laptop's fan controller to take full control of the fans when the workload and thus temperature increases.
Thank you very much for any help and for your time.
Happy Holidays