After a lot of searching I finally found a good schematic for a PH monitor. The author of the original schematic is richard.c from the electro-tech-online forums.

Because I didn’t want to play with chemicals and etch the board myself, I decided to order a PCB from a fab house. However, since there is no PCB design I had to recreate the schematic in Eagle and put together the PCB. So I sat down one day and within a couple of hours I had the schematic done in Eagle. There was a little error in the original schematic and I’m glad I spotted it before I ordered my PCB . After another hour or so I had the PCB layout ready as well. I used BatchPCB to order a prototype board as their prices are reasonable for low volume orders. The only thing is it takes about 6 weeks to get your PCBs back, but that’s fine with me.

So last Friday I got this in the mail:

I had ordered the parts from DigiKey in advance (see table below) so I sat down and started populating the board. The build is pretty straight forward, all through-hole parts and not too many of them:

I found a “bug” with U4, but at least it wasn’t fatal enough to render the boards useless. I had reversed VO and VI so all I had to do to fix it is mount the IC on the bottom. This is now fixed in the latest version.

So I connected my PH probe and calibrated the board using a 7pH fluid (turn RV2 until VPH reads about 1.709V) and then a 10pH fluid (turn RV1 until VPH reads about 2.441V). I have not connected the module to my arduino yet but that should be straight forward as well. Only trouble is I’m running out of space on my 16×2 LCD…

Eagle Project files: (includes board, schematic, gerb files and gerp process file)
http://joro.geodar.com/code/PH%20Amp%20eagle.zip

Note: I have not followed any design rules (because I don’t know them) for the PCB, only kept the analog and digital parts separate as per Richard’s advice.
BOM:

PH Amp BOM