Here is a simple Circuit to automatically switch ON and control a speed of a Fan. For the simplicity of the circuit we have not included control to threshold level.
Circuit Diagram of Automatic Fan Controller
this project uses IC LM35 as a sensor for detecting accurate centigrade temperature. Output voltage of this sensor is linearly proportional to the Celsius (Centigrade) temperature. This sensor use the fact that, as temperature increases, the voltage across a diode increases at a known rate. Output of IC is 10mv/degree centigrade for eg if temperature is 45 degree then the output of sensor will be 450mv or 0.45V
Output data of sensor is applied to a current amplifier circuit and feed to a low power DC motor. We have made current amplifier by using a general purpose NPN transistor. I have used BC548 as shown in the circuit diagram. If you are using other NPN transistor, then make sure about pin configuration. Transistor will start conducting when base voltage reaches to 0.40 V (40 Degree). You will see fan start to rotate and at 0.60V (60 degree) it will be at full speed. At this voltage transistor is fully conducted means resistance is low .
Part List of Automatic Fan Controller :
IC LM35, Any general purpose NPN Transistor eg BC548, BC187 etc. Low power DC Motor you can easily get from old thrown DVD Player or Tape Recorder, small fan Blade, 10 Ohms or 4.7 ohms resistance, battery etc.
If you are using other NPN transistor, which is not mentioned above then please make sure of pin configuration, I mean base, collector and emitter pin. If you are using CPU fan or SMPS fan which is used in desktop PC then use 12 Volt power supply and remove 10 Ohm resistance, instead directly connected to positive rail.