Temperature controlled DC fan

admin August 30, 2008 10 Comments


Here is a simple circuit based on two transistors that can be used to control the speed of a 12 V DC fan depending on the temperature.A thermistor (R1) is used to sense the temperature. When the temperature increases the base current of Q1 (BC 547) increases which in turn decreases the collector voltage of the same transistor. Since the collector of Q1 is coupled to the base of Q2 (BD 140), the decrease in collector voltage of Q1 forward biases the Q2 more and so do the speed of the motor. Also, the brightness of the LED will be proportional to the speed of the motor.

Circuit diagram with Parts list.



  • The R1 can be a 15K @ 20°C ,N.T.C  thermistor.
  • The M1 can be a 12V,700mA fan motor.
  • The capacitor C1 must be rated 25V.
  • The circuit can be powered from a 12V PP3 battery or 12V DC power supply.
  • Assemble the circuit on a good quality PCB or common board.


  • sandeep parihar
    May 4, 2015

    it is working quiet efficiently plz send me a brief notes on its working….

  • avnish shrivas
    September 20, 2014

    hey i jst wnts to knw that can we use it for celing fan by doing some modifications. if possible then what will be those modifications…??
    otherwise can u tll me the future use of this project?

  • saai tej
    August 14, 2013

    actually the collector current increases when base current but as per above ones collector voltage decreases when base current is increased

  • sasidhar reddy
    April 7, 2013

    why we are using capacitor across the dc fan ,mention it ,and applications of “temperature controlled dc fan”…..

  • Anthony
    February 3, 2013

    I saw this circuit and I’m wondering what R7 and R5 are for.

    • miraat
      April 20, 2015

      TO FILTER ,,in order to reduce jerk

  • Shriraam.s
    November 18, 2012

    I trust this circuit more.Last time i tried with IC TDA7274 which become
    robust and good. I like to adopt an 12 V DC fan into my television
    for cooling purpose. Motor speed need to be adjusted.
    Well this circuit is adequate and i gonna to try this.

  • Sulaiman Koroma
    February 12, 2011

    Need notes on the functions of the individual parts of a temperature control fan, how the circuit operates and how it will stop operating to assist me understand the project.

  • February 10, 2011

    need ablock diagram of the same [temp. ctr fan] to assist me understand the project.

  • January 20, 2010

    Need an infrared thermometer cabale of measuring 1000′ to 2000′ F


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