Picture of 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 amplifier.it will reduce the unwanted usage of the cooling fans. please rate it.........

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

ourichie9 months ago

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.

sherk29 months ago

Do we have to heat thermistor to make this circuit work

RITHINROBERT9 months ago
afelix m1 year ago
how to connect the circuit with cooling fan..?
steps392 years 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
pcline23 years ago
About how much current do you think this circuit would be able to handle?
sooraj619 (author)  pcline23 years ago
a 12v 3A power supply (max) the 741 IC burned after this
pfred23 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)  pfred23 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.
coolkids4 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