I've simplified the system further to lend itself to massive expansion. The planter bottle has been modified to eliminate the need for interconnecting hoses as well as make it accept ANY airtight, and I recommend RIGID, reservoir. It sort of reminds me of a mosquito and works much the same way. As the air pump increases the pressure inside the reservoir, the nutrient sol'n is forced into the body of the mosquito. When the air pump is off, the resulting lower pressure allows gravity to return the nutrient to the reservoir and empty the body of the mosquito.
Modify as required and desired. For simplicity, I've quickly built a two planter unit out of stuff I had on hand and have ignored any efficiency issues. This is more to show the concept and although it "works", this one will not be used as a unit itself. I have also not filled the planters with bags of grow medium and plants for this same reason.
It'll be real short, I promise.
Step 1: What You Need
One air pump
1/4 inch air tubing as required
Two pop bottles
Two 12 inch pieces of 1/2 inch CPVC pipe with one end cut at an angle
1 ea 5/8 inch drill bit
1 ea 7/32 inch drill bit
1 ea 9/16 inch drill bit (please pretend that's what's in the picture)
One drill (not shown)
Step 2: The Mosquito
Cut the bottom off the bottle.
Place bottle upside down on pipe.
It kinda resembles a mosquito, don't ya think?