The figure below shows a circuit of an automatic temperature controlled fan using 555. The circuit will control the speed of the fan automatically according to the temperature. You can use any type and size of 12V DC fan marked 1 to 1.5 ampere ratings. The circuit can be used at any place where a 12V supply
will be available.
The circuit is using a 555 timer IC which is a well know timer IC available in 8 pin dip package. The circuit is using a 10K NTC thermistor for sensing the heat. The 12V fan is derived through a BD139 transistor. A diode is used across the fan to save the transistor from the back EMF of the fan coil. You can also experiment with different values of NTC thermistor and resistor R1 to make the speed of the fan as desired on any temperature.
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