Simplest and probably cheapest is a toilet float valve. Incoming liquid can be gravity fed. Mount to bottom of tank with silicone. If you are handy, a simple bat handle switch can be mounted to furnish low voltage control.
Ward

A more expensive way might be to use another minimalist pump with two water level switches and mount them at the on and off levels you want.

set your valve so the open position is lower the the reduced level while the pump is running. Also, don't fill your rez to the rim. leave enough volume for a bit of play in the max liquid level.
I don't believe you want the water to be topped off every time the rez drops 1/4" or so. It should only be coming on when the level drops to an unworkable level (or something close to that)

You can try putting whatever is feeding the float valve and topping off the res on a timer to run once or twice a day in the early morning or late evening when the plants are not being watered and the res is full. US Plastic has float valves.

He really hasn't said how big this system is and it makes a difference whether it is a small hobby project with corresponding economical parts which he is just worried he just can't get to refill all the time or needs to go away for a few days, or if it or some large project.

The slick solution to this might be what he has in mind, although personally (like Grizz and sdgrower) I can't imagine what reason is to maintain it always perfectly topped off and I think all he needs is a too low point at which it refills a certain amount, where the tank has enough capacity above the refilling level to receive the system volume if it all returns.

Slick solution:

Normally open solenoid valve connected to the same outlet as the pump, so when the pump is supplied power, the solenoid closes and the problem is solved in this way.

Something just like this for a hobby set up, although "normally *OPEN*". Maybe the seller has a normally *open* one. They are relatively economical and work gravity fed slowly which is perfect to allow the system to pick up the slack when the pump turns off and water returns.

1/4" SS 120V AC Solenoid Valve STAINLESS / VITON S20V (note: you want the normally open version of this or similar. Perhaps sprinkler companies have some too.

He has a 1/2" normally open 12V solenoid valve here listed for $13 which is specifically gravity fed and thus has a higher flowrate which may or may not be more desirable. This particular one being 12V would require a $3.50 adaptor to run off 120V if you wanted to use it with line voltage, but you should be able to find one that meets your preferred flow rate and pump voltage without needing an adaptor, I'd give them a call if you want to go this slick route and can't google up something more specific along these lines that you can just drop into your system to block the topping off water connection off while the pump is working. It sounds like they can meet all kinds of specs and the quality does look decent.

Normally open but 12V, this one is gravity fed which gives a faster flow rate. You would estimate the flow rate that best suits you.

This post was edited by PupillaCharites on Fri, Sep 27, 13 at 19:38

You can try putting whatever is feeding the float valve and topping off the res on a timer to run once or twice a day in the early morning or late evening when the plants are not being watered and the res is full. US Plastic has float valves.

Sorry for repeat post above....

A little more detail on how some of my systems are setup.
I run a recirculating drip system that runs a few minutes per hour. The first reservoir has a pump on a timer that feeds the plants. The first reservoir also has a float valve which is fed from a second res with half strength nutes. The pump in the second res is on a timer set to run early each morning before the plants are given their first watering. It keeps the main reservoir topped off and the half strength nutes keep the EC from increasing too much. My leachate PH typically climbs, so my top off water has a lower PH to offset that as well. It took a little trial and error to get it right, and it's never perfect but it is reliable and keeps the plants getting water/nutes.
This setup still allows me to make daily adjustments if I prefer, but also allows me to go for days without adjustments or adding water.

sd, What a hydro wonderland if you can ever just sit down and have a cup of coffee watching it do its stuff and then 'audit'. Sounds like you've semi-automated adding by guesstimation pH down, water one day and nute the other about as much as the plants will let you nap on them anyway since something else always needs a little attention;-) I'm envious in a very good way.

Then you do use two pumps and two timers...You could accomplish the same without both the second pump and its timer in the auxiliary res, by replacing them with one of the normally open solenoid valves (from $15 to $25), which would keep the res auto topped always, the only caveat being aux. res needs to be higher up. I wonder if you would have a reason to prefer one design to the other if you thought about both options?

Guy,

sounds like you have some experience in methods and means already. i have a simple float valve set up on an "infinite" supply for top off. its straight RO for my low pressure aeroponics runs 24/7. when it turns off, i do understand your dilemma of having the back-flow raise the water levels in your sump.
I make sure my drain pipe is above the waterline when the pump is off.

my only practical answer was to keep the system running at all times.

Would this work?

http://www.ezautofill.com