What I want to do
As explained in an earlier research note, we're looking to measure the conductivity of water using a 555 timer. The ideal frequency range of our setup is around 1 kHz, and we're going to need to be able to measure oscillations of this frequency with an Arduino -- which might require some tricks. I'm going to build this research note out as I go along, but for now, I'm just collecting a bunch of "Reference" links online that seemed relevant (sorry it's all jumbled, for now) ...
(Source for this fantastic depiction of the 555's internals.)
I think I've got working code now! I'll need to come back and document this more fully, but the wiring is more or less as described in the linked-to-research-note above, and the code is fairly well commented, here:
1 μS/cm is then equivalent to about 0.64 mg of NaCl per kg of water.
two metal screws about a bottle cap away in water seem to have a resistivity of ~ 160 kOhms
"151" cap is 0.015 uF