hey, im working on the same idea, the main dilema is that with a float switch, when your system does its normal flood the float valve will activate allowing your reservoir to overfill.
so i came up with THESE TWO ideas below not sure which one, i plan on testing both when i get all the parts.
1- have float valve in the main res. allowing water to be gravity fed from secondary nute res. above. the resolution to the ebb and flow overfill problem is to place a normaly closed soleniod inline with the gravity feed tube. this solenoid would be on a timer and will only activate opening the gravity feed tube, once or twice a day at times when the system is not in flooding. so in effect when the res. needs nutes the float valve will open, calling for nutes, which would only flow from the gravity fed res. only at times when the solenoid is open controlled by a timer which would coinside with the flood schedule. I have decided to use 1/4 tubing and solenoid, for i only have a 10 gallon reservoir, and i timed 1/4 tubing gravity feed at approx. 1 gallon per 3 minutes, so the flow will be plenty fast at 1/4 inch.
2- this method is a little simpler but with that comes a little less control. First you have to measure the average amount of nutrients your system consumes a day. for instance i am currently consuming 1/2 gallon a day. now you get a water pump, and see how long it takes the pump to pump 1/2 a gallon, you will have to reduce the flow, which you could do many ways , i choose to get these clamps which pinch the tube and are fairly adjustable, and i also reduce the tube down to 1/4" .
Then you simply put the pump on a timer to pump for however long it takes to pump 1/2 gallon.
You can also put in a float valve in your main reservoir to stop the flow of nutes when it reaches max, probaly much safer. with this i am planing on putting a T or Y rite before the feed reaches the main reservoir to allow fluid to return to the secondary tank when the float valve is closed.
I have found many of the parts to do these experimental nutrient doser, on web site http://www.usplastic.com/catalog/default.asp
. they have been great they have everything plastic and are fast and reliable.
So i plan on trying these out within the next two weeks a will post the results, if anyone has any other ideas or revisions, please feel free to add on to this post, i really want to build a better moustrap with this one. When i have asked for help from my convention hydro friends they all had the same reaction and told me it was not a good idea to use such a contraption on my dear plants, for it is more than likely to flood or overflood them ,than to make maintaining them and god forbid leaving them for a long weekend easier and safe for the plants and safe for me..