Operational Amplifier as a Schmitt Trigger
The Schmitt trigger works as a threshold switch that turns on the output voltage at a certain input voltage which we call the upper threshold voltage (VUT) and turns off the output voltage at a certain input voltage lower threshold voltage (VLT).
Non-inverting Schmitt trigger
The function of a Schmitt trigger can be well shown by the example of a sine-square converter:
Non-Inverting Schmitt Trigger Function Switching Behavior Hysteresis