Sorry for the late reply and thanks for pointing that out! I have to fix that.

I finally used a 10 MOhm potentiometer to adjust the sensitivity which is dependent on the LED intensity and the aperture in the printed measurement cell. I would suggest using just a resistor for the LED (with more than 220 Ohm) and then only a 5-10 MOhm potentiometer and then play around with it because a lot depends on your build and print!

Let me know if it worked for you!

I forwarded this to Anna an Juan since they were the ones working on the pH control

Thanks!
Well, we just used google slides to put it together because we all worked on it remotely. If you want to draw great illustrations, I highly recommend you to use a vector graphics software. Inkskape for example is great! It is open source and free and has so many great plugins.