This float switch is designed as a "level kill switch" where you want to turn something off when the water level gets too high - like the main pump of the display tank if it is about to overflow. It can also be used as a "water available switch" where you only want a pump on if its underwater - for instance your top off pump shouldn't go on if there's no RO water in the reservoir. It would just run dry. The switch setup is very easy to build (drill several holes) and mount (three screws). And uses very cheap but reliable parts. It runs your 125V devices directly. And the best part is the materials can be as cheap as $8.
You could use this for auto top off, but I don't recommend it. The switch point is very narrow (more on this below). My ATO Float switch page describes a better solution for ATO applications.
This image shows the main parts of what you'll need. Click for more detail:
The main disadvantage to a switch that has such a short lifespan is you're relying on to keep from flooding. So you are forced to build in redundancy.
The key part of this float switch is a "snap action", 120V, 3A rated switch. I originally started out to build this using an ordinary light switch, but soon realized the flaw in using those switches - short cycling the pump or solenoid. Short cycling (turning on and off rapidly) severly shortens pump or solenoid lifespan not to mention wastes power. Most reed type switches suffer from this same operation.
The "Snap action" forces the switch to change position completely before the internal contacts are toggled (from off to on, or on to off). This ensures that the water level will move a fixed amount before the switch changes.
I found a waterproof snap action switch which I think will work well in a corrosive environment. I don't intend to use it underwater although it looks like you could. But the point here it to keep the wires (and mechanics) away from the water.
The switch is an Omron Part # D2SW-3L2MS. I purchased mine from mouser electronics.
If you don't want the little roller on the end you could try the D2SW-3L1MS or D2SW-3L3MS. They are the same switch, they just have a different lever arm.
The switch is designed to close under 2.1oz of force. I measured 1.5oz on mine. The distance of lever travels from on to off (and vice versa) is about 1/16". This is the switch's major challenge. If you were to use my design on an autotop off you would gain rock solid sump leveling at the cost of pretty frequent pump/solenoid cycling. I have designed a variation of this setup to increase the on/off distance of the float. I built this version of the float switch as a water level detector related to my gravity assisted pump where the water level is not directly affected by the pump I am running. So I don't get switching during normal operation.
Here's the parts again. Click for more detail.
Starting with the 1/2" schedule 40 pipe cut off a length around 5". Drill using a 7/32" bit around 4 holes adjacent to one another until you have a slot long enough to house the switch.
Using a utility knife cut out the slot so that the switch fits very snuggly in the slot. I made is so my switch is so tight in there, I don't have to glue it in. It will just sit in the slot and wont wiggle out.
Here is the switch in the slot when you are done.
Next drill two 3/16" holes next to each other on both sides of the pipe. Round out the holes to the long elipse as shown here. You want the elipse wide enough to allow the screw to slide in and out easily and long enough to allow the pipe to hinge up and down a few degrees.
Next drill two 3/16" holes on each side of the "Tee" as shown.
Drill two 3/8" holes on 3/4" outlet side to allow access to screw in the mount as shown. Basically, this hole is big enough to put your screwdriver through so you can get to the screw when you go to mount it.
Insert the 1.2" long pipe into the 12oz soda bottle. This fitting should be so snug that it requires a fair amount of force to get it started. This works out nicely since it holds strong enough not to have to glue it. This means later you can adjust the bottle up or down to change the water level switch point to fine tune it. Insert the 7" long 3/4" pipe into the "tee." Insert the other end of the 1/2" pipe into the 3/4" thin walled pipe. The 3/4" pipe works as a sleeve for the 1/2" pipe to slide up and down easily. The force of the float is directed upward and on to the switch arm - closing or opening the switch. Here's the contraption assembled ready for installation. I inserted the screws in the holes so you could see how it mounts into the cabinet roof.
Screw the "Tee" to the roof of the cabinet with the 3/4" wood screws directly over the spot on the water where the float will be positioned. Slide the switch arm into the "Tee" until the switch roller is centered in Tee's length. Mount the switch arm into the cabinet roof using the 2" screw. Do not screw this screw in all the way. Leave at least 3/8" or so of space between the cabinet roof and the swith arm pipe so the arm can swing to let the switch close.
Here is the float in my overflow box.
The wiring is easy.
Hook up the hot line of your outlet through this switch. Your on your own for doing the electrical. If you are using this as an top off or high level kill switch, use the black and red wires. Leave the blue unconnected. See the data sheet for more wiring info. I used black and blue for my application. I wanted normally open. I only want the pump on when it's intake is enveloped with water. Pumps aren't very happy when they run dry.