I'm going to add some notes to this tomorrow, but just wanted to jot some stuff down.
Recipe for a conductivity meter that blinks an LED at a frequency that is inversely proportional to the conductivity of a solution.
Here's the wiring diagram:
And here's some Arduino code to monitor the output (more explanation tomorrow), as well as measure the temperature (an important variable for conductivity):
A preliminary test showed a nice, solid, repeatable relationship between conductivity and the average 555 timer pulse duration (y axis is average pulse duration in seconds, x axis is index of data points, sampled every few seconds -- the probe was initially placed in a higher conductivity solution, then then briefly in a lower conductivity for three separate stretches (indicated by the three higher plateaus)) ...
... more soon!