This circuit is a Schmitt Trigger, a type of comparator, built using a 555 timer chip for some reason. 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.
This circuit uses the fact that a 555 output goes high when the trigger input goes below 1/3 Vin, and the output goes low when the threshold input goes above 2/3 Vin.
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