valuable info from another thread:

Quote Originally Posted by User_Defined View Post

......As for sensing the Z height, I think that the suggestion of using a strip of foil on the side of the top reservoir is absolutely brilliant. I would likely use copper foil as it is easy to solder wires to, and comes in standardized thin tape rolls.

This would only work with relatively thin dielectric walled containers, and would likely need a calibration step. You would also need to have another wire inside the reservoir to complete the circuit. Once calibrated, the capacitance should be linear with salt water height.

I don't know what the Peachy uses for electronics, but the way I would implement this is to use a 555 timer as a capacitance meter, and then send the square wave signal to the computer. Audio works best since you can just pulse modulate your drip received signal so every droplet interrupts the current state of the Z height. Imagine two people, one constantly expressing their detailed undying love towards the current height of the reservoir, and another impulsive and insensitive colleague occasionally interrupting them to announce when a droplet has been received.

Schematic: http://www.eleccircuit.com/simple-ca...-using-ic-555/
(One of many out there)

I would go for the newer 2MHz capable 555's since the capacitance may be in the pF range.
........

-UD