On 3/5/2010 9:25 AM, yamanoor sairam wrote:
> Hi, I am trying to interface an ORP probe with PIC 16877A opamp.  I
> adopted the design for the ORP probe interface from the link below:
> www.diy-labs.com/projects/arduino-phorpduino-shield The product was
> done using an Arduino Board. The eagle design files were available at
> the link. I used the schematic available for the TL072 opamp for my
> PIC16F877A. The board that I had design using Express PCB is
> available in jpeg format at my blog page:
> http://mechatronicscraze.wordpress.com/pic-orp/ The schematic that I
> had used is also available in the same page in *.jpeg format I am
> facing a few issues with my design. When I interface my probe with
> the opamp, I see no changes in the voltage displayed from my LCD
> port. However, I am able to adjust the gain of the opamp. When there
> is no input/input to the opamp, I see that the impedance adjustment
> opamp ouput / pin 1 of TL072 sends out 4.2V signal to the offset
> adjustment opamp. Ealrier I was able to adjust the gain between 0V
> and 4V. Now, I am able to adjust it only by a few V. How do I find
> out whether my opamp is working fine? How do I calibrate my opamp? I
> have plans of connecting my opamp to a DC RPS and make it read the
> voltage I give to the opamp. Is that possible? When I adjust the trim
> pot, I see that the output of the opamp is offset as a negative
> voltage and 0.000V is read by my PIC. I have a very poor knowledge of
> opamps. All I want is that I want my PIC to read my ORP voltage. ORP
> sends out voltages between 200 and 800mV. If it works fine, I could
> introduce ozone into my tank and kills the infectants. My basic
> question is whether my opamp is working fine. Have I adopted a wrong
> circuit? Also I used a 39K resistor for the impedance adjustment
> instead of 40K.
>
> Sairam
>
>
>

Some observations...

Your opamps are powered up with +/- 5V.  The ouput stage has a 5.1V 
zener diode to limit positive excursions to 5V and negative signals to 
about .7V.

Your first stage has a gain of +5; the second stage has a gain of -1. To 
test your opamp, build a resistor divider, say 10K to 1K.  The 1K goes 
to ground. The resistor junction goes to your orp input without the orp 
probe.

Now if you connect the 10K to +5V, you will get about .45V as your 
input.  The first stage then should produce around 2.25V.

The second stage will invert this but will be subject to the offset pot 
setting.  Set the offset pot to zero volts and you should see -2.25V at 
pin 7, not at the VORP point.

Now move the 10K from +5V to -5V.  You should now get the same output 
voltages except reversed in sign.  If so, your opamp is working correctly.

As far as reading ORP signals from 200mV to 800mV, you MUST connect the 
probe such that it presents a negative voltage to the BNC input jack. 
This is because of the inverting second stage.






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