I did mine following the Melev's writeup http://www.melevsreef.com/plumbing/auto_topoff.html and I improved it a bit (hopefully ) with 2 changes:
- instead of running 110V through the float switch I used a 5V power source and a relay to control the pump's outlet, so only 5V goes through the float switch. The power source is from an old cellphone charger and the relay i bought it cheap from ebay.
- I use 2 float switches: both are placed in the sump, one is for min level and one is for max level. Both are in "close" position by default and "open" when the water pushes the floating cylinder up. The one for max level sits outside of the water, 1-2" above the normal water level. That is the safety switch in case the other one fails. The valve for min level sits at the water level and this is the main switch that closes and opens as the water level goes down and up. They are both connected in serial and act as a double switch for the 5V power going to the relay.
I bought the float switches and the relay from ebay. I bought the outlet box, outlets and a cable outlet plug from home depot.
I use the JBJ auto top off on my tanks. I used a DIY kit before that had a 9v battery running that shorted out and caused a fire. Never again. LOL. Very reliable system and can be found on sale sometimes for $75-$80. Normally $99.
I went one step further and bought the Spectrapure "Ultimate Top Off" (or whatever they call it). Ridiculously expensive ($200) but no problems, flawless operation, I love it. It feeds into my kalkwasser mixer. Love it!