"The entire circuit consisted of several parts: an opamp-based salinity-measuring circuit, a linear positive/negative power supply, a digital voltmeter, and a peltier device temperature controller.
In order to save time debugging (inevitably) flawed bread or perfboarded circuits, I designed custom circuit boards in Eagle CAD and etched them at home with ferric chloride. I won’t post my .sch/.brd files as that would make it too easy to copy this project for Science Olympiad, but I will share schematic image files and board image files (if you already know how to reCAD them for etching in Eagle, chances are, you’re already beyond this guide)."
[Source] HackedGadgets via A Digital Salinometer
