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[on:]

Hi all,

I am working on a project that requires input from a PH probe. I have found a few example circuits built around opamps but most output a -ve below PH7 and +ve above.

Could this be read by the Arduino analog in?

Could I modify this circuit - http://www.66pacific.com/ph/ph_3.htm - that uses a 7V reference ground? I only need to measure PH values between 5 and 9 so perhaps I could change the reference ground to 2.5V so that the range fits the 5V input range of the Arduino?

Sorry that this is so open ended but I am trying to get to grips with this

[Reply #1 on:]

Sorry for the following poor reply... but better than nothing?

a) Have you decided on the pH sensor that you will be using? If so, posting the details might inspire/ enable answers. A URL to its web-home for specs, etc would be useful

b) I am no expert, but I THINK that you CAN tell op amps to shift their outputs. The demo circuits on the web are "tuned" for pH 7= 0 volts. higher = positive, lower = negative are set up that way becuase it is convenient... IF the op-amp's output is being sent to a negative-capable voltage display. But (I think) there's nothing inherent in the fundamentals of op amps to prevent you from configuring one to read 0v for pH 5, 5 v. for pH 9.

Hope this is....

1) Right!

2) Encouraging.

Sorry I can't be certain or more informative.

Oh yes. Also. Break the problem up. I suspect your pH sensor generates a very weak voltage, let's say 0 - 14 mV for pH 0-14, capable of sourcing 0.1 mA? Once you have the actual figures for your sensor, you can start looking with Google at op-amp circuits IN GENERAL... you don't need to search for an article about using an op amp with the specific voltage source you are working with.

When I've tried to work with op amps and things like the Arduino, I've stumbled on the following "What if".... "What if the op amp starts sending out a voltage outside of the range that is accepatable to my ADC? (In the case of the Arduino, outside of 0-5 volts)" Can I prevent it? Does it "matter" if I don't? (I suspect negative voltages would be a Bad Thing.

Let us know the answers when you find them!

[Reply #2 on:]

On the subject of Op Amps, I recently found a really helpful Op Amp tutorial that explained them in a way I could understand for the first time. :-)

--Phil.

[Reply #3 on:]

Thanks for the reply

My understanding of the PH probe itself is that it puts out -ve for acids and +ve for alkali and 0V for neutral.

Looking at the circuit I found on the web it has been designed to shift the ph readings so they are all +ve and used an opamp to give a 'handy' range where 7V is neutral and the ph scales well with 1V = 1 on PH scale.

Could I just use a voltage divider to bring this in the range of the arduino?

[Reply #4 on:]

Here you go:

http://www.vernier.com/probes/ph-bta.html
it is an analog device
data for values http://www2.vernier.com/booklets/ph-bta.pdf

pinouts
http://www.vernier.com/probes/specs/pinout.html

[Reply #5 on:]

Have you looked at the circuit at [url][/http://www.66pacific.com/ph/simplest_ph.aspx[url]? It's as simple a pH meter as you can make. You can just add another op-amp to increase the voltage and/or the polarity of the output. The IC Op-Amp Cookbook by Jung has lots of good circuits for that.

[Reply #6 on:]

A question:
wont that be affected by salt and impurities in sea water?

thank you.
Sincerely, J. Terramoto 8-)