Here is a picture of mine. It's 15 gallons total when I use the two buckets, but usually I just use the 5g acrylic resivor with float valve. No relays, switches, or anything else to worry about. It is simply a gravity feed Kalk ATO. Drips at the exact rate water is evaporated, it does not flush like some ATO's do.
I just had a 5 gallon acyrilic tank made to my dimensions. I would have used a 10 gallon glass, but I had only 8 inches clearence behind my tank. I simply ordered two adjustable float valves. The bottom of my ATO sits on the top of my sump. The float valve sits in the sump and adjusts the water as used. You can also see another float valve in the top of my ATO. I use this when I hook up the other two buckets if I leave town for a week. Works great, extremely simple, cost me $28. If I used a 10g glass tank it would have been $15.