Water Level Prototype

After doing a bit of research, I settled on a capacitive water level probe. It uses an insulated wire that dangles into the tank, which forms one plate of a capacitor. The water forms the other plate of the capacitor, so the capacitance will vary with water height.

The capacitance only varies by a few pF, but it is measurable! The trick is to measure the time it takes for the capacitor to discharge through a 1Mohm resistor:

Here's my prototype test circuit in action, it reads digital 20 for an empty tank, and 40 for about 30cm of water. I should be able to increase the accuracy beyond that.