Well, there’s the kinda easy way and the not so kinda easy way. It looks like you’re going with the not so kinda…
First a few pointers. Schematic drawing convention has power supply lines at the top, ground at the bottom. If there’s a negative voltage bus, that goes below ground. Inputs (sensors, controls) on the left, outputs (fans, transistors) closer to the right. Signal flow from left to right, top to bottom. Little things like that make it much easier to read your diagrams.
OK, now back to the matter at hand (been listening to vintage Snoop Dogg recently…)
This circuit: http://www.cedarlakeinstruments.com/Sha … ontrol.png is a differential fan controller that switches the fan on when the battery sensor is hotter than the ambient sensor and it’s an example of a somewhat “harder” way to do it. It was designed for the parts the person I was working with had on hand. Don’t remember if I actually built it or not!
The “easy” way is with a semiconductor temperature switch that can control the fan directly. Problem is most of them these days are surface mount and don’t lend themselves to easy breadboarding and I can’t think of any parts off the top of my head that would do it in a single package anyway.
So let’s talk concepts you can Google and then come back for more detailed help.
A comparator (e.g., LM339) is a useful device to take a sensor input and a setpoint and switch the output on or off as the sensor goes above or below the setpoint. A comparator can generally drive a FET (sf has logic level FETs: https://www.sparkfun.com/products/10213) easily and the FET can switch fairly large motors. Comparator output by itself won’t switch much. It’s certainly possible to do this all with just a few transistors, but honestly it’s not worth the effort. Comparators are [cheap.
You can use a thermistor since you have them on hand, or a linear semiconductor temperature sensor that gives a predefined output, e.g., 10 millivolts per degree F and is easy to set. For one-off’s I prefer semiconductor sensors since they are just so easy to use (argh! tried to link to a blog post I did on sensors but realized I haven’t published it yet).
So, take the output of an [LM34 sensor (10mv/deg F), feed it into the + input of an LM339, give the - input of the '339 0.90V, and tie the output of the 339 through a 1k resistor to +12V or whatever regulated voltage is available (you want it regulated so you can use a simple & cheap voltage divider or potentiometer to generate the setpoint. Regulated voltage is coarse, but Good Enough) and watch the output. When the temperature is below 90F, the output will be off, when it’s above 90F it will be on. Now tie that output to the gate of your FET, ground the FET’s Source pin, and connect your fan between +12 and the Drain pin et voila: fan goes on and off with temperature.
I’d draw a diagram but CircuitLab is apparently no longer free. grrr.](http://www.bgmicro.com/TRNLM34DZ.aspx)](http://www.bgmicro.com/ICSLM339.aspx)