Temperature sensor circuit based on ∆V BE ... AN-460 LM34/LM35 Precision Monolithic Temperature Sensors, Texas Instruments, Application Report SNOA748C, May 2013.

You also need to select the transistor by the type of fan that you use. In my case I used the well-known BD139 transistor and a 9V battery to provide power to the fan and transistor. The LM35 ...