Get a thermostat that switches at the right temperature range.Hook up a low-voltage (12 to 24 volt, typically) circuit through it to a relay which is controlled by that voltage and which can handle switching the voltage/current of the swamp cooler. You can use cheap bell wire to put the thermostat a fair distance from the relay, just as is done with any other household thermostat; it's not going to have to carry much current. Check that the thermostat causes the relay to switch.Hook the relay's switched connections into the power line for your swamp cooler. This side of things *DOES* carry lots of voltage and current. Make sure you're hooked to the right pair of output terminals -- typically one set is normally open and another is normally closed, and which one you want depends on which way your thermostat switches.Personally, I'd recommend building the relay into a box with a power cord on one side, terminals for the thermostat on another (or the thermostat mounted directly on the box, take your pick), an AC outlet on the third, and the relay and 12v power supply inside. Then you can just hook up the thermostat, plug the box into the wall, plug the swamp cooler into the box, and turn the swamp cooler on... and the Right Things should happen.
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