Use a Sentinel or CAP controller. I guess in your case you could use the temp sensor on the outside of your room. Get a feel for your room's average temp. If you constantly find it to be around 80, have the controller kick the intake fan on when it drops below that.
like Abusive said. Also you can ad a backdraft damper to help with unwanted air being able to get in the grow room. They also make electic backdraft dampers that can open and close when you want them to.
You can get a cheap thermostat at any grow shop. They cost around 50 for just the cooling thermostat.
What I want is more than a set point.I also have motorized damper to control my air cooled lights
I run the lights at night with AC going 24/7. I want to take advantage of the cool night and early morning air. The when it get warmer than the room the fan would turn off. I could use a set point but I want to automate it a bit more. When I get HomeSeer to use the temp sensor it will be able to do it with a software solution. I'm looking for a hardware solution.