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Originally Posted by g8wayg8r
I've checked my wiring and my notes.
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Notes! Wow, you're more organized than me.
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Originally Posted by g8wayg8r
I started out with the hysteresis on the non-inverting input but it altered the pH reading due to the additional voltage.
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Connect the selector switch for the display directly to the output of the LF411. The new resistor between the LF411 and the LM339 + input will shield it from the hysteresis voltage. 220 ohms sounds low, try 1k as a starting point.
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Originally Posted by g8wayg8r
The 1K pull-up resistor on the out put of the LM339 gives me almost 12V.
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If you still have a pull-up resistor on the hysteresis circuit/LM339 output, remove it. That will throw the ratio for sure.