Looking For Dual Temperature Controller For Fan System

I am looking for a controller that could power at least one fan based on comparing outside temperature to indoor temperature. If outdoor
temperature is below indoor temperature and indoor temperature exceeds some threshold, I want to bring in the outside cooler air.
Is there any control that will do this cheaply?
--
Will
westes AT earthbroadcast.com
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Put a J thermocouple outside, a second one inside. Connect their negative wires together. Connect the other two ends to an ON/OFF temperature controller ranged for 0 - 100 deg C with the cold junction compensation disabled. Put the set point to = the threshold temp at which the fan has to come on.
You will have to play around with or work out which of the two + wires go to the controller's + terminal.
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Will: Just noticed the indoor temp threshold requirement. Resubmission below:
Put a J thermocouple outside, a second one inside. Connect their negative wires together. Connect the other two ends to an ON/OFF temperature controller ranged for 0 - say 100 deg C with the cold junction compensation disabled. Some controllers offer this feature. Put the set point to near zero (= the indoor/outdoor temp difference at which the fan has to come on)
Use a cheap wall thermostat set to the indoor temp below which you don't need cool air coming in. Connect its contacts to inhibit the fan when the thermostat shows "cool enough".
You will have to play around with or work out which of the two + wires go to the controller's + terminal. And which thermostat contacts to use - N/O or N/C.
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