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I am working on a project similar to this post. For the circuit diagram on that post, what are the function and working principle of the upper amplifier? I know that the lower amplifier is used to magnify the input signal from the CO2 sensor. Thanks! The design circuit copy from the other post

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  • \$\begingroup\$ The answer was in the module in section b. comparator, you guys, you not even read the post of your question ! \$\endgroup\$
    – R Djorane
    Commented Feb 19, 2016 at 11:40
  • \$\begingroup\$ The reason you don't see the functionality is that the schematic is poorly drawn \$\endgroup\$ Commented Feb 19, 2016 at 11:55

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The upper opamp is being used as a voltage comparator with its reference voltage being supplied by R1.

The lower opamp has a non-inverting DC gain of 8.5, and when its output voltage is more positive than the voltage on U1-5, U1-7 will go low, turning Q1 OFF and allowing INPUT to float.

However, when U1-6 goes less positive than U1-5, U1-7 will go high, turning Q1 ON, which will pull INPUT down to Q1's Vce(sat), about 300 millivolts above ground, give or take.

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In that circuit, the upper OpAmp is taking an input reference voltage on its non-inverting input and the output from the lower opOpAmp on its inverting input.

This forms an OpAmp Comparator, with the Potentiometer setting the comparator's reference/threshold voltage.

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  • \$\begingroup\$ A potentiometer is not a varistor. \$\endgroup\$
    – EM Fields
    Commented Feb 19, 2016 at 10:20
  • \$\begingroup\$ @EMFields Thank you, I mis-spoke. Answer corrected. \$\endgroup\$ Commented Feb 19, 2016 at 10:28

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