Need help designing pH meter

Thread Starter


Joined Sep 27, 2006
Hi, I have been designing a pH meter.
I have the circuit for the meter part (a 10-step bargraph) LM3914
But the electrodes output 0-300mV based on the pH.
The IC above has a 1.2V minimum full scale voltage.

Can someone please recommend an OP-Amp that runs on a 3V or 12V battery, not requiring any negative voltages, that can
input=0 - 300mV
output=0 - >1.2V (I'm not sure about the max. voltage from



Joined Apr 20, 2004

You're in for an uphill battle. A calomel electrode outputs something like 59.6 mV/pH - but it's negative and positive on either side of 7. A pH of 14 is over 400 mV - 414 comes to mind. Acidic pH's are negative, with 0 being -414 mV.

As long as you only want to measure alkalinity and not too far past pH 10, you're ok. Any op amp can run with the negative power pin grounded. You will need a large input resistance to keep from loading the electrode. If you think 10^13 is enough, try an OPA134PA. They're about $2.75 from Digi-Key.