For Burning Man 2014 me and a friend built a Figjam Swamp Cooler. Since swamp coolers requires water to be re-filled when they evaporate, and since no one wants to wake up in the middle of a much needed sleep to do that, we built a mechanism for auto-refilling the bucket. We installed a float switch, and together with a 12V relay attached it to an auxiliary pump. When the water level would go below the switch’s threshold the relay would switch the power from the main pump to the aux pump and water from a reservoir will get pumped into the bucket. It worked wonderfully and we barely had to touch the system for the entire burn.
Over-engineered Swamp Cooler Board
Eran Rundstein's Blog 