Reposted from Don Blair's earlier note.
This circuit modifies the 555 Blinky LED circuit to blink an LED at a frequency that is inversely proportional to the conductivity of a solution. We built a bunch of them during the 7/12/14 Water Quality Workshop @ MIT Media Lab and tested them out!
How to build it
Salt solution: We used 100 ml water in five different solutions
1 - No salt (distilled water). 2 - 100 mg (low salt). 3 - 425 mg (medium salt). 4 - 1100 mg (high salt). 6 - 5000 mg (brine - similar to salt water).
Water bottle setup: Cut the top off and fill your water bottle to 100 ml
Build your sensor Two screws in the cap, roughly 1 cm apart and attached to alligator clips -- this is your sensor and goes in place of R2
Pictures of built sensors
4 (~groundwater runoff) vs #6 (~seawater) w/ piezo instead of LED -- one whole tone difference!
Output to Arduino
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)) ...
Tone generator dance machine version - swap output to an audio cable - add a potentiometer to drop the voltage to a safe range for your computer - limit max voltage 0.5V (without no pot the output is 2.8V) - feed it into a tone generator -- make some tunes!
Capacitor swap analog synth version - swap 10uf cap for .01uf cap - add a piezo speaker in parallel with your LED - instead of clicks, you hear distinguishable tones! - one whole step between salinity #4 and #6 -- noticeable difference! (note: with the smaller capacitor the LED will not blink for either circuit -- it's blinking too fast and looks like it's permanently on)
See also: open water wiki