With this simple circuit you are able to control the speed of a DC fan according to temperature measured by a temp sensor. It’s an ideal addon for your PC cooling fans to eliminate produced noise.
On the first site two circuits are displayed. The first circuit is a simple version using LM317 and a temperature depended resistor to detect temperature. The voltage applyed to the fan is proportional to temperature. The second is a more advanced one using LM35 as temp sensor and four OPAMPs to achieve linear response of the output according to temperature measured.
On the second site you can find a Temperature controlled 12V dc Fan circuit using a Thermistor. All designs are selected for their simpicity, so the choise is yours!
Temperature controlled DC Fan – [Link]
Temperature-controlled 12V dc Fan – [Link]