Temperature Controlled computer Fan

Hi there I am wanting to build I believe a fairly simple circuit. I want to put a probe in my computer that turns on a fan when it senses 40 degrees C and turns off when the temp drops below 40 C. Simple on or off I don't want it to be variable speed or anything.

I have some temperature probes from a temp display panel for my computer. The simple probes give temp info by resistance (obviously).

Temp probe give a resistance of about 10k at room temp
I want the circuit to turn on when it has a resistance of about 5.7K
I would also like to have a pot to adjust this voltage as well

I believe the 12 volt fan can draw about .5 amps, but probably best to assume .75 amps to be safe. If its easier I can just use a relay to handle the higher current.

I just started messing with Tiny CAD and I have attached what I have thus far, its not much..

I believe I could use a transistor to do this, but I have no idea on how to do the calculations for anything. Any help is much appreciated!
thank you very much,

Seth

After much research I was able to find this diagram:

**broken link removed**

I think I could just do this instead. The part of it that I am unsure about it the zener regulator. Does it need to be 10v or does it matter?

Seth

[edit] His notes are helpful:

"If you just happen to have a regulated 12V supply handy, leave out the 10V zener diode, use a short circuit for R5, and just use the circuit as is."[/edit]

KISS,
temperature switch, thermostat.

Hi

If you want to build that circuit here is the PCB layout in Eagle 5.6.0. it is zipped.

I use this circuit and have so for three years, it is excellent.

Cheers
Andrew

thanks Andrew, the circuit looks great. What kind of set-up do you use for the diodes and what type of diodes do you use?

Thank you,
Seth

Hi

I happened to have two Gl 9242 laying about so used them. One broke and ended up with one GL9242 and one 1N4141. I have noticed that the circuit does not like two or one for that matter 1N4141.

Dont forget R8 needs calculating according to the article.

I use the fan for a completely different application, it is set to come on at ~28°C and is at full speed at ~35°C. This is nice as it is very quiet at the lower temperatures.

Cheers
Andrew

dear i want some ideas of circuit projects?
plz give me some ideas also give me circuits diagram.