What is the simplest way to toggle a relay with analog feedback?
I think you should consider using an analogue component known as a comparator. It would compare the signal from your thermistor with a set-point set by a potentiometer and tthen output high or low depending on which is higher.

Wiki is quite useful on this and worth a read. You can also make a comparator from an op-amp and realistically this will work just as well for your application.



Vin is compared against Vth. Vth is set by R1 and R2 with a slight influence from R3. In your application, R1 and R2 would be a potentiometer, maybe 10k and R3 would be typically 1Mohm. R3 provides hysterisis that guarantees a certain level of protection against the comparator output "chattering" when Vin nearly equals Vth.

Rload in the circuit can be your relay coil.