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Decision making before micro processors


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i need a circuit to turn a relay on and off.
the relay controls 12 volt fans.

the control condition is heat, which comes from my refrigerator, and the fans vent the heat away from the cooling fins.

so when frige comes on i would like to have the fans come on shortly thereafter.
and when the frige goes off i would like to have the fans go off.

i could even do it without a processor using an a/d converter, but i dont even want to use an a/d converter, seems like overkill.

the circuit should be simple , maybe using an op-amp ,maybe.

thermistor with changing resistance..

but how do i turn a changing resistence into what i need ?
driveing the relay isnt a problem assuming i have a signal to drive it with.

so i have a changing resistance ,and i am working with 12 volts i think the relay is 12 volts also

Thank you.

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