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Raspberry Pi Class
It would be even cooler to make it real 3D, instead of lighting up the whole row as a single sampling data.. I suggest to add a few msec time delay and variable in arduino to store each of the row separately, and perhaps the front LEDs would be the real time, and then starting to shift to the rear, giving it real 3D look..
From my experience, usually it's not that difficult to figure out the amount of water it needs, with proper tool (syringe with needle). I usually just shine a light into the hole while I'm slowly refilling it, just right above the element/sponges/whatever it's called. You'll see it inside the hole. and then wait for 30mins or so for the water to get absorbed (it it has been dry for a while), and then put more water, same as before, just right above the element.