Instructables
Picture of heat activated cooling fan
25062011020.jpg
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.........
 
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Step 1: Parts list

Picture of 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    
afelix m6 months ago
how to connect the circuit with cooling fan..?
steps391 year ago
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?
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
pcline22 years ago
About how much current do you think this circuit would be able to handle?
sooraj619 (author)  pcline22 years ago
a 12v 3A power supply (max) the 741 IC burned after this
pfred22 years ago
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.
sooraj619 (author)  pfred22 years ago
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.
coolkids3 years ago
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
sooraj619 (author)  coolkids3 years ago
(removed by author or community request)
sooraj619 (author)  sooraj6193 years ago
the motor that works from 9 - 12v can be used, or use a 9v relay
sooraj619 (author)  sooraj6193 years ago
yes