This is taken from my local site.
I have had this sitting around for a time and thought that I might as well finish it. I built the latching circuit from a guy who put it on RC; I have since forgotten his name. I still have the schematic.
Today I made the bracket that will hold the float switches. One of the floats will set on top of the other. The way it works is the top float will drop and nothing will happen until the lower float drops then the RO system will start moving water via a solenoid valve. When water raises the lower float nothing happens until the water raises the upper float, then the water is shut off. The switches run on 12 volt DC and the solenoid sees 120 AC.
I made the circuit from parts found at Radio Shack. They consist of a relay, pc board and screw type terminals. Soldering one is pretty easy if you understand the schematic. It took me about 30 minutes.
I have been using a timer, solenoid and float valve to regulate my auto top off. I know that my tank uses about 4 gallons of water, through evaporation, My RO/DI is rated at 200 gpd, and I do not get that much because of water pressure. I know that I make over 5 gallons an hour, so I set the timer to stay on for an half hour in the morning and afternoon. When the water in the sump if full the float shuts off the water. This is not ideal because I still put the RO/DI against the float valve which builds TDS, The new system will shut the system down when the sump is full and not trickle through the float valve as water evaporates.
The bracket and float switch mounts are made from ¼ inch acrylic. I heated them until I could bend, the bracket was bent into an upside down channel, and the mounts are bent into an L.
I also ran the wire needed to the tank and through the basement to the RO/DI. I need to pick up some nylon screws and nuts for the bracket. So I am finished tonight.
The next pic shows the black wires joining to telephone wire. This wire was the cheapest I could find in the length I needed. It has 5 insulated wires that are insulated together. I stripped and soldered the 4 wires keeping the color configuration on paper. This is needed to make sure the wires make it back to the pc board in the order needed. This wire follows my line from the basement ro/di to the display upstairs. I used about 80 feet. This wire also only sees 12 volt and very little amperage, just enough to power the relay coil.
This pic is of the float switches in the sump on the bracket. I did a water change to set it up. The lower switch powers the relay coil when it lowers. This is about 7 gallons of sump water. I lowered it as low as I could before the sump return pump sucks air. When the upper switch raises it cuts off the power to the relay coil and water stops flowing. The mechanical float is set to stop the water about 1/2 inch above the float switch. The wires are a bit misleading, the lower one is about 1/4 of an inch from the top float, and the big wire in between the mechanical float and the switches does not make contact with anything.
The last pic is of the box and solenoid valve. The wire going into the top of the box is that flat telephone wire mentioned earlier. There are two other wires that come in one is from a 12 volt converter and the other is a 120 volt wire. The 12 volt converter is to energize the relay coil, I did this because I like the idea of a weak 12 volt current in the tank, 120 volts from the house grid does not sound good.
It is constantly dripping fresh water through the valve, so not really any time for salt to build up, and I believe the part that rotates is plastic, so rust is not an issue there. 
