[g8wayg8r]

Ok. I'm starting anew again using an quad op-amp as a comparator circuit because the voltages from the LM339 are driving me nuts. 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. Both go into another op amp that does the controlling. The pH goes into the inverting input and the set-point into the non-inverting input with positive feedback to give it some hysteresis. That output is inverted to turn the transistor on when the pH is high and off when it low. The transistor circuit is rather simple and I have only had success using either a power transistor or a reed selonoid as an emitter follower. 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. Sometimes I wonder if I would have done better starting with something programable like BasicStamps. Oh well. The good new is, however, is that it appears to work. The best I can do now is plus and minus 0.03 pH units.

I think I'm going to stick with this basic set-up and work with it from here.

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.