# building pH controller

Discussion in 'General Electronics Discussion' started by vega77, Feb 1, 2011.

1. ### vega77

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0
Feb 1, 2011
I'm planning my pH controller now.
Due to the high impedans in the pH circuit and the low voltage i cant connect it directly.

I have taken a look at national semiconductor reference design as shown in attachment.
And that seems ok for me.

But since the pH probe gives out - voltage under pH 7 and + voltage with pH over 7 i must somehow convert that voltage into a 0-5v signal.

Any idea how i could do that ?

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2. ### Laplace

1,252
184
Apr 4, 2010
In your reference circuit, the first op amp provides gain for the probe signal for a range of +/- 700 mV or a total range of 1.4 volts. The second op amp provides signal inversion, range scaling and level shifting. Your final output signal will have a range of 5.0 volts and be level shifted so a zero volt input will give a 2.5 volt output.

The basic equation for an inverting op amp circuit with non-zero reference voltage at the '+' input is (as I calculate it):
Vo = -(Rf/Ri)Vi + (Rf/Ri+1)Vref

So R9/R4=(5.0/1.4) and (2.5)=(R9/R4 + 1)Vref. Just choose R4, R9, and Vref correctly.

3. ### vega77

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Feb 1, 2011
I helped me alot.

i didnt include the components value in my pervious picture.
There are two not so commen resistor values R5 36.5k and R6 619k
could i use 33k or 39k for R5 and 680k for r6 ?
what impact will that have ?

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