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The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. I designed this circuit to use 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.

A litle link for al of you

http://sound.westhost.com/project42.htm

Step 1: Creating a Small Suport for De Circuit

I made maind oute of a CD cover!! and glued some screws tu hold the circuit!

Step 2: The Circuit Board

I uset to controle my pc case fan's

Step 3: Aplication

pc mounting
Forgive me if I'm just not getting it, but if the diode will turn it on with just your fingers, won't the fan be on pretty much any time the computer is on? Isn't that what most case fans do?
I'd love to see more detail about how you arrived at the circuit. I realize some of it is basic "get a book on op-amps" territory, but the instructable would be more instructive if you included some links to data about diodes' thermal behavior, any references you used while figuring out the op-amp circuit, and so on. How did you arrive at the idea of putting several diodes in parallel? Does the hottest one dominate the reading, or do you get an average?
Very interesting idea. I also like the use of the CD case cover as the mount . Did you build the electrical components from scratch? What other tools and or glues did you use. Can you explain the steps more in depth and explain exactly why and how it works and why or how you came up with the idea. Please forgive me, I am not trying to be rude, but it looks as though English is a second language for you. A lot of words are misspelled, I could help you fix them if you like. Or I can suggest a couple of things to help you. Download Firefox Web browser, it has a built in spell checker, use MS words spell checker , or use dictionary.com Nice instructable.

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