Hi,
Still progressing in my project I read here and there that the cheapest way to build a pH-meter was to use a high-input-impedance operational amplifier (Building the Simplest Possible pH Meter • 66pacific.com).
So I went to digikey as usual and found this guy: ST LF351 http://www.st.com/stonline/products/literature/ds/2152/lf351.pdf.
The package is great, and I could use a 12V input (at least from what I understand from the datasheet).
The only costly part would be the probe it-self, but here you cannot do anything against that (by the way how often do you need to use pH buffer solution when the electrode never moves from the same kind of solution: the fish tank)
Since it would be to measure if the pH is between 7.5 and 8 ... do you think it would do the job? I do not really care about a .01 precision ... +- 0.3 would be enough ...
Does someone tells me if I am on right path with that?
Thanks!
Looks like it'll work, but be aware that your opamp requires a differential power supply, so you need +/- <18V power supply. Also be aware that you can't use one half of the differential supply to power something else or it will be out of balance (you can't use the +<18V side to run LEDs or something)
I'm not sure that powering it off of 2 9V batteries will be the best idea, you're better off making a simple split power supply to run it so the voltage output is consistent.
I am going to use this kind of method PH Meter - ProjectWiki, using this device: ICL7660A.
I am still trying to figure out the final schematic.
Finally a fast and reasonably cheap alternative that allows immediate interfacing between Arduino and pH probe. The pH/ORP Adapter - 1130_0B at Phidgets costs 30$ and has ideal inverted signal, 525mV offset and 4.75 gain. I have to admit: I tried the self-build option and failed.
Just wire up with Arduinos 5V, GND and an analog input and done.
I find the easy'st way to get good sensor values is to buy a shity low budget device. Then hack it and use its sensor input/processor output.