This circuit is a Schmitt Trigger, a type of comparator. It measures the input to see if it is above or below a certain threshold. The threshold varies to make it less likely that the output will switch rapidly back and forth due to a noisy input near the threshold.
The input is a noisy 40 Hz sine wave, shown in the first scope. The threshold is shown in the second scope. The third scope is a graph of the output versus input. Note that the threshold goes down whenever the input rises above it, and vice versa.
The two 10k resistors form a voltage divider that put the threshold voltage (the + input of the op-amp) at 5 V. But, the output of the op-amp is also connected to the threshold input through a 100k resistor. This causes the threshold to be raised or lowered slightly depending on the op-amp output. -- Credits: Mr. Paul Falstad.
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