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Fan speed controller

Fan speed controller

This proto implementation has components installed for two sensors, controlling two groups of fans. The switch is just for safety backup: if something would break or not function as intended the switch provides full fan speed independent of other circuit operation.
After some bad experiences with commercial fan speed controllers in the PCs in my home, I decided it would be easy to build a better one myself. Indeed, after a few evenings, I obtained these results….


  • Three temperature sensors are supported, each controlling the speed of a (group of) fan(s).
  • A small potmeter for each sensor allows to choose the actual temperature above which the fan starts running. Higher temperatures cause a linear increase in fan speed.
  • When the measured temperature decreases, the actual fan speed is decreased only very slowly (taking several minutes), to avoid control instability.
  • An additional potmeter provides a minimum fan speed, independent of temperature.
  • The circuit is built with very few components, thanks to a low-cost PIC 16F676 microcontroller. This controller takes care of A/D conversion of the analog inputs, proper signal filtering, and the creation of PWM output signals to drive the fans. The PWM driving of the fans is a fast on/off switching with variable duty cycle, so the output transistors will remain cool even when driving many fans.
  • A bicolor LED gives a visual temperature indication by gradually changing color from green to red.

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