In Arizona, we will average 100+ degrees from June to September. Our challenge and concerns are around shade and keeping our chickens cool. When the chickens are constantly exposed to this weather they get stressed and can even result in loss of life. I have had a misting system set up in my coop area for a couple of years now and have always had the idea of automating it with the temperature as the variable to set it off. I finally go to it and here is how I did it.

Items Used:
Attic Fan Thermostat - $26.50
3/4" inline sprinkler valve - $13.47
~3ft of 16ga 3-wire electrical cord - $1.86
15A 125V Male Plug - $2.99
Plastic 6" x 6" x 4" junction box - $11.78
3/8" sheet metal screws - $1.18
24v Transformer - $14.96
Cheap extension cord - I had an old one so i cut off about a foot of the female end
3/4" x 1/2" PVC male adapter - $1.34

Tools Used:
Drill
1/8" drill bit
Wire cutter/stripper
Wire nuts
Phillips & Flat screwdrivers
3/8" drill bit
Hack saw
PVC primer and glue.

I started by lining up the thermostat and transformer in the box to make sure everything fit nicely. The thermostat has some mounting holes so I drill a couple of 1/8" holes in the bottom of the box and secured it with 3/8" sheet metal screws. I then drilled holes in the junction box to line up with the outlet holes in the transformer. I stripped one end of the 16ga 3-wire cord and attached the male plug and stripped the other end to attach to the thermostat input. I cut about a foot of the female end of an extension cord and stripped its end to connect to the thermostats output. I could then plug the 24v transformer into the extension cord. I cut off the plug on the the transformer and wired it with the sprinkler valve.

Everything wired inside the junction box.

Thermostat cover on.

Old ball valve:

New sprinkler valve:

I mounted the junction box under the roof of the roof a foot or so above and a few feet away from the mister nozzles. The junction box also has a lid to make it water tight.
After Thoughts:
I probably could have just used an A/C thermostat, but I didn't take the time to look into it and I'm not sure how well it would hold up with the outside heat and blowing dust. If you decide to make one of your own try that and see how it goes. I think that would save you from buying both a thermostat and transformer. I think the A/C thermostat would do that all in one device? Thanks for looking!

Future plans:
If I can easily find one I'm going to put, in line before the thermostat, a photocell that will shutoff the system at dark. The temperature can easily stay above 100 degrees after midnight. This would help to keep the area from becoming a mud hole when the chickens are sleeping.

Thank you everyone for the advice and help!