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My thoughts are that it is used to set the trigger point where the trigger fires. That is to say, if the voltage exceeds the reference set point, the trigger will then fire.
To independently control the upper threshold and the lower threshold, some Schmitt trigger gates have 2 variable resistors: * a voltage-divider potentiometer, where the center tap selects the "average threshold"
* a feedback variable resistor, which adjusts the amount of hysteresis (the difference between the upper threshold and the lower threshold).

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15y ago

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