Introduction: Heat Activated Cooling Fan

this a circuit based on the op amp ic 741. this circuit can be used for cooling fans inside pc or an will reduce the unwanted usage of the cooling fans. please rate it.........

Step 1: Parts List

1 x LM741 IC
2 x IC base (holder)
1 x 10k thermistor 
2 x 10K resistor(brown black orange)
1 x 1k resistor(brown black red)
1 x 680r resistor(blue grey brown)
1 x 10k variable resistor (preset)
1 x BC327 transistor 
1 x 1000uf 25v electrolytic capacitor
1 x 2200uf 25v electrolytic capacitor
1 x a cooler fan or a 12v DC motor
1 x common circuit board

Step 2: Circuit Diagram

complete the circuit according to the circuit diagram, the 10K pot  is used to change the sensitivity or temperature range,  here the transistor BC327 act as the switch, this circuit works from a 9 - 12v DC. a filter 2200uf capacitor is provided for a dc source is used from a step down transformer or eliminator.8 pin IC base is used for the IC741. the transistor base or holder is made from the 8pin ic base(holder) by breaking it into two halves of 4pins each and using the three pins for the transistor. this will  reduce the burning of transistor when soldering. the LED shows whether the circuit is on or off it is optional      

Step 3: Conclusion

this a circuit which helps to reduce the unwanted usage of cooling fans. the thermistor used must be soldered fast without burning the thermistor.never touch the thermistor's black part with soldering iron it will damage the thermistor. instead of cooling fan you can use a relay to control other equipments. hope you like the circuit... and please rate the circuit ,and if you have doubts about the circuit just comment and please rate the instructable