It’s the simple circuit, control the speed of a Fan according to the temperature.
Working: In this project we used IC LM35 as a sensor for detecting accurate temperature in centigrade. Output voltage of this sensor is linearly proportional to the centigrade temperature.
This sensor used the fact that, as temperature increases, the voltage across a diode increases at a known rate. Output of IC is 10mv/degree centigrade for e.g. 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 e.g. BC548, BC187 etc. Low power DC Motor you can easily get from old thrown DVD Player, racing cars, toys or Tape Recorder, small fan Blade, 10 Ohms or 4.7ohm resistance, battery etc.
You can use any NPN transistor, taking care of pin configuration like 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.