Do not use this circuit. The transistor will overheat.
Yeah, it's pretty simple at this point. The op-amp is a
schmitt-trigger to prevent oscillation, but this also means it toggles
around the set temperature point instead of settling at a constant
level. It's also abit simplified - there should be a buffer between
the thermistor and the input to the op-amp. The transistor also needs
a BIG heatsink, and possibly some current-limiting 10-20 ohm resistors
as well. Don't underestimate the power dissipation values -
run the numbers, or get burns on your thumb like me.
The component values aren't too critical... the thermistor is 10K at
25'C and decreases as temperature rises, and the op-amp I used was an
LM324 but just about anything could be used.