Done! Well, not quite, but the basic blocks are there. I took a video of an LED being dimmed by a PWM according to a variable resistance input, but it's pretty poor quality, so I won't bother posting it.

Now for the fun stuff. Below is a description of what I plan to build. Before anyone asks, the difference between a 'simple' controller, and an adjustable/variable one, is a couple of potentiometers and some code; ie, very little additional cost in more hardware.

Inputs:
12v switched
ground
temperature sensor
AC on/off

Outputs:
PWM Trigger wire to fan
Power led
Output that can drive an LED in the cab to show fan status.
Status LED on the controller (indicate temp sensor not found, etc)

Settings:
2 potentiometers (variable resistors).
- Turn on temperature
- Full speed temperature

Each has a labeled temperature range.

Functionality:
At the turn on temp, the fan will turn on to 60%.

It will increase linearly to 90% on at the high speed temp. As the temp drops, so will the fan speed, potentially below the turn on temp, when it will turn off again. It will go several degrees below the turn on temperature to avoid rapid on/off at the turn on point.

When AC input is high, fan will go to 90%, after a 15 second delay. After the signal turns off, the fan will continue at full speed for 30 seconds.

If input pots are set to non-logical state, the fan will always be at 90%.
If the temp sensor is not detected, the fan will always be at 90%, after a short delay on startup.

Screw terminals for all input/output wires (18awg).

At this point, it won't be completely sealed from water, but the board will be sprayed to protect it from occasional splashes. Just don't hose it down

Currently aiming for a price around $50, without a temp sensor from Summit (I'll provide links to the sensor(s) I recommend).

Anything else?


*Edited to add/clarify features