This is just a rough idea at this point, I haven't had much time to check on using different materials and to refine the whole concept, as well as any testing. The image is a also just a rough picture because I didn't want to spend a lot of time on it in case it's not something you can use.
When the
pump turns on, the water is pumped into the top reservoir. As the water rises in the top reservoir it flows down the open
drip system to the plants. The simple float in the lower reservoir is gravity feed from the top reservoir. This will insure that the water level in the lower reservoir wont get to low and run the pump dry. If the drip system doesn't drip fast enough, the overflow from the upper reservoir to the lower one will make sure that the water level will never back up and get to high in the top reservoir. As well as help make sure the pump will never run dry also.
The simple float in the top reservoir is attached to a fresh water line to keep the water level correct (sorry I forgot to show the fresh water line in the image). This can be a water line directly from the pluming or a gravity feed line from a third reservoir. A third reservoir could have just fresh water or a diluted
nutrient solution to replace what was used by the plants and/or evaporated. In either case you wont need to check the system daily and it could be left untended if you go on vacation.
The simple float is just an open and close valve like the ones found in a toilet tank and in swamp coolers. There are many different designs of these float valves. I haven't been looking around for them, but those are the two main uses I know for them. Also, it's not in the picture, but I would design the placement of these float valves so they don't actually go through the wall of the reservoir, but sit inside the reservoir instead. That would make it more adjustable in case you want to use the reservoir for other designs later. How to do this would depend on the actual float valve used but the base would need to be heavy enough to stay put underwater.
The adjustable height P.V.C. tube is just a short tube with a couple of elbows, this makes it easier to set at the right height. This should be placed at the height of the water level you want the drip system to start flowing at. The simple float should at this level also. This way when the pump turns on it will start flowing through the drip system right away, and when it turns off the float will make sure the water level does not drop below this. Other than the 2 float valves the only moving part is the single pump.