Operation of the proposed temperature controlled fan may be understood as follows:
The speed of the DC motor alters as the temperature increases which is converted into a proportionately rising voltage and applied between its terminals.
To measure temperature thermistor (R1) to be placed as close as possible to where you want him to sense temperature is used.
In the diagram, one can see that the thermistor (R1) and the resistor (R2) are employed to form a voltage divider network. It is recommended that the value of R2 is around approximately one tenth of the value of R1.
As the temperature thermistor value decreases it causes the transistor Q1 to saturate harder proportionately.
Since the collector of Q1 is connected to the base of Q2, the voltage at the base of Q2 also responds to the above and decreases
The voltage decreases at the base of Q2 which becomes saturated harder, causing the collector-emitter voltage (VCE) to lower thus intensifying the voltage on the collector terminal of the motor.
The maximum speed of the motor will be slightly less than its rated specification.
To add to this, it may not be crucially necessary for precise circuit operation, to know the temperature in order to control engine speed an LED may be used as given in the diagram. This LED will proportionately get brighter as the engine speed increases.
R1: 15K thermistor
VR1: preset 22K
Q1: 2N2712 (NPN) or equivalent
Q2: BD140 (PNP) or equivalent
M: Motor DC brushed or brushless
Note: The DC motor can be different from a computer motor. Make sure that the current rating of the motor does not exceed the rating of the transistor Q2. (maximum current 1.5 amps). It is recommended not to exceed 1 amp and use sink.