555 Temperature Controlled PWM

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: -

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!