I had to wait for the thermistor and other bits to arrive through the front door, so took some time to try and find some software to help mock up the stripboard layout. I found one for Windows 8.1, but its not perfect. I might just try to do it in Word or something. Anyway, the final bits arrived, and got installed into the breadboard, and amazingly, it WORKS!
Functionally, I measure the temp every few seconds, and map anything between 30 and 50 degrees to a duty cycle of 90 to 255 PWM output. (90 is about the slowest this fan will go). I’ll document the code in another post, when I’ve got a circuit to show. I do a bit of a trick with the constrain statement to get the transistor to kick in.