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Originally Posted by g8wayg8r
I've feed the output from the pH meter and the set-point reference into seperate op-amps that are wired as a voltage follower.
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Probably unnecessary, but as long as it works and you're comfortable with it, go with it. I see no functional problems in your arrangement.
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Originally Posted by g8wayg8r
That output is inverted to turn the transistor on when the pH is high and off when it low.
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What's the purpose of R15/R16?
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Originally Posted by g8wayg8r
I've also noticed that I can clamp the output using two zenor diodes and it has a profound effect on the reading and hysteresis. So, I've not figured out how to make the controller and the meter independent of the power handling portion of the circuit.
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I'm not sure exactly where you're placing the zeners, or the value. Let me know and maybe I can explain why this is causing a problem.
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Originally Posted by g8wayg8r
Sometimes I wonder if I would have done better starting with something programable like BasicStamps.
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It wouldn't have simplified much. Although I admit, I have a secret desire to see a cheap DIY pH meter with USB interface.
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Originally Posted by g8wayg8r
Cobra: I really appreciate the help. I can go months and learn less. Anyway, I'm still not certain about the panel meter wiring. Maybe I'm not clear. I've got an on-on two position switch. One wire is attached on the non-inverting input and the other to the inverting input. Another wire goes to the panel meter to read the voltage. I know somthing is missing.
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No problem on the help.
If it's working, nothing is missing.
Meter input attached to center switch terminal, inverting/non-inverting inputs attached to outer switch terminals, and that's it.
Other notes: D3 in the diagram is backwards. D1 may be backwards also, but I'm not familiar with your use of an LED as a voltage-control device, so I'm not sure.