Reposted from [Don Blair's earlier note.](http://www.publiclab.org/notes/donblair/07-11-2014/simple-555-conductivity-meter) ###Project This circuit modifies the [555 Blinky LED circuit](http://www.555-timer-circuits.com/flashing-led.html) 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 ](http://www.publiclab.org/notes/donblair/07-11-2014/water-quality-workshop-july-12th-mit-media-lab) and tested them out! ###Diagram [![555conductivity-nocap.png](https://i.publiclab.org/system/images/photos/000/005/236/medium/555conductivity-nocap.png)](https://i.publiclab.org/system/images/photos/000/005/236/original/555conductivity-nocap.png) [![FI66X99H7996R0X.MEDIUM.jpg](https://i.publiclab.org/system/images/photos/000/005/238/medium/FI66X99H7996R0X.MEDIUM.jpg)](https://i.publiclab.org/system/images/photos/000/005/238/original/FI66X99H7996R0X.MEDIUM.jpg) ###How to build it Salt solution: We used 100 ml water in five different solutions [![20140712_160519.jpg](https://i.publiclab.org/system/images/photos/000/005/239/medium/20140712_160519.jpg)](https://i.publiclab.org/system/images/photos/000/005/239/original/20140712_160519.jpg) 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). [A good explanation of conductivity and salinity](http://www.fondriest.com/environmental-measurements/parameters/water-quality/conductivity-salinity-tds/) [We used this page to compare salinity to real-world samples](http://www.fao.org/docrep/t0667e/t0667e05.htm) [![20140712_143829.jpg](https://i.publiclab.org/system/images/photos/000/005/242/medium/20140712_143829.jpg)](https://i.publiclab.org/system/images/photos/000/005/242/original/20140712_143829.jpg) Water bottle setup: Cut the top off and fill your water bottle to 100 ml [![20140712_150326.jpg](https://i.publiclab.org/system/images/photos/000/005/246/medium/20140712_150326.jpg)](https://i.publiclab.org/system/images/photos/000/005/246/original/20140712_150326.jpg) 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 [![20140712_143049.jpg](https://i.publiclab.org/system/images/photos/000/005/241/medium/20140712_143049.jpg)](https://i.publiclab.org/system/images/photos/000/005/241/original/20140712_143049.jpg) With a variable cap in place of R2 for testing [![20140712_144749.jpg](https://i.publiclab.org/system/images/photos/000/005/243/medium/20140712_144749.jpg)](https://i.publiclab.org/system/images/photos/000/005/243/original/20140712_144749.jpg) Putting it all together [![20140712_143028.jpg](https://i.publiclab.org/system/images/photos/000/005/244/medium/20140712_143028.jpg)](https://i.publiclab.org/system/images/photos/000/005/244/original/20140712_143028.jpg) Modifying the design [![20140712_160313.jpg](https://i.publiclab.org/system/images/photos/000/005/245/medium/20140712_160313.jpg)](https://i.publiclab.org/system/images/photos/000/005/245/original/20140712_160313.jpg) 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): https://gist.github.com/dwblair/f0baedb7b8e155b804ae 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)) ... [![conductivityjuly14-2.png](https://i.publiclab.org/system/images/photos/000/005/240/medium/conductivityjuly14-2.png)](https://i.publiclab.org/system/images/photos/000/005/240/original/conductivityjuly14-2.png) ###Mods **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 ](http://www.publiclab.org/wiki/open-water)