Here is a simple circuit to automatically turn on and control the speed of a fan according to temperature.
For the simplicity of the circuit we have not included the control at threshold level.
Work: This project uses IC LM35 as a sensor to detect accurate centigrade temperatures. The output voltage of this sensor is linearly proportional to the Celsius (centigrade) temperature. This sensor uses the fact that, as the temperature increases, the voltage across a diode increases at a known rate. The IC output is 10mv / centigrade for eg if the temperature is 45 degrees then the sensor output will be 450mv or 0.45V
The sensor output data is applied to a current amplifier circuit and fed 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 another NPN transistor, make sure the pin configuration. The transistor will start to drive when the base voltage reaches 0.40 V (40 degrees). You will see the fan start to rotate and at 0.60V (60 degrees) it will be at full speed. In this voltage transistor is completely driven, the resistance of means is low.
List of part of automatic fan controller:
IC LM35, any NPN transistors of general use, for example, BC548, BC187, etc. Low power DC motor that you can easily get from a DVD player or recorder, a small fan blade, a resistance of 10 ohms or 4.7 ohms, battery, etc.
You can use any NPN transistor, being careful of the configuration of pins as base, collector and emitter. If you are using the CPU fan or the SMPS fan that is used in the desktop PC, use the 12-volt power supply and remove the 10-ohm resistor, instead directly connected to the positive bar.