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this a circuit based on the op amp ic 741. this circuit can be used for cooling fans inside pc or an amplifier.it 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    
<p>Is this on stripboard? If not can it be produced on one? And can the small variable resistor be replaced by a small potentiometer?</p>
<p>I am needing to step down from 12vdc to 5vdc (want to use a small 15mm 5v .06mA fan due to space issues. Adding cooling fan to a Yaesu Ft-1900r ham radio) could I use an l7805 somewhere to charge a 5v cap? I am totally new to this so forgive my ignorance. </p>
<p>Do we have to heat thermistor to make this circuit work</p>
how to connect the circuit with cooling fan..?
Hi, I would like to increase the voltage range of the device 5 - 12V so that the fans turn off completely when the PC is cold and then build up to max at about 50 deg is this possible and which resistors control the lower voltage please.
will a 100k thermisistor work?<br>
yes it will, but you better increase the resistor with which it makes a voltage divider to 100k as well. With a comparator like this the actual value of the resistors is less important than their relative value
About how much current do you think this circuit would be able to handle?
a 12v 3A power supply (max) the 741 IC burned after this
What does the 2200uF capacitor do in the circuit? I hate using those spring IC sockets too. They cause so many problems in projects. They're actually more expensive than ICs often too. If I have to socket an IC I use machined turret sockets. They're a lot nicer.
2200 uf capacitor is used to reduce ac ripples from a 12v dc transformer when connected as per supply, in case of batteries it is not necessary
OK. I didn't think it would do much good with a battery.
can it work just like a cutof in your ac like you set temperature as 24c on your ac and when the 24c is achieved it stops cooling and the blower mode goes on so if excess heat is present in the sorounding then the motor starts working to cool the excess heat

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