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A pH Measurement & Control System for the Planted Aquarium

Page 5

Inverter and pH Display

The output of the amplifier stage is 1 volt per pH unit, but it is inverted, so a pH of 6 produces an 8 volt output, and a pH of 4 gives 10 volts out.  The inverter's sole task is to change polarity of the amplifier's signal before it is sent to the pH display so that the pH reads correctly.  Note that the non-inverting input, pin 5, is not connected to the minus supply but to the reference ground.  47k resistors were used to keep the loading on the amplifier stage light to help limit heating and drift.   Another 10 uF unpolarized capacitor, C400, has been included here to reduce noise.  C401 is a bypass capacitor.  It's leads should be short, and it should be kept close to pins 8 and 4.

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