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I am trying to build a temperature controlled 555 circuit to drive a cpu fan (used for cooling my entertainment center)

I built this using slightly different diodes and MOSFET, and it works great (until you hook up the power backwards and get that distinctive pop of and poof of smoke from a blown up 555). Circuit: -

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anyway, now I want to make it temperature controlled. I have a few tmp36 sensors laying around as well as an AD592 and a AD22100, so I can use whichever will work best. how could I go about converting this circuit to vary based on the temp? Let's say lowest speed below 75 F and full out above 100, but I would love to learn, so if you could explain the calculations, that would be great. I feel like this should be easy, and I am kind of just missing something, but who knows.

Thanks!

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    \$\begingroup\$ You'll find it a lot less painful to use a MCU. Some even have thermal sensors built-in. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 19 '13 at 2:07
  • \$\begingroup\$ yeah, I know that would be easier, but I want to learn how to do this, plus, I'd rather not waste an arduino or spend more on something when I already have these parts around \$\endgroup\$ – lkrasner Nov 19 '13 at 2:13
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    \$\begingroup\$ thanks, but once again, now I really want to know how to do it with a 555, sometimes it's not about the easiest way, I want to learn \$\endgroup\$ – lkrasner Nov 19 '13 at 2:16
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    \$\begingroup\$ Instead of "wasting" an arduino (which are, in my rather jaded opinion, a waste of space in the first place), you could use a PIC10F or 12F with a PWM output and an ADC input to implement a PID controller in much the same space as a 555. \$\endgroup\$ – markt Nov 19 '13 at 7:15
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You can accomplish temperature feedback by connecting the analog voltage out pin of the TMP36* to pin 5 of the 555. A temperature increase will increase the voltage at pin 5 of the 555, this increased voltage will increase the "on" time of the 555 as I've outlined elsewhere. This will, in turn, increase the voltage that the fan is experiencing and thus its speed! You will need to tune the size of the potentiometer and capacitor to suit the application.

*Connect the other two pins to \$V_\text{cc}\$ and ground according to the datasheet.

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