DIY PC Fan speed controller

Jos writes – “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….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.” [via] – Link.