This is a simple square wave oscillator using a 555 timer chip.

A timing interval starts when the trigger input ("tr") goes lower than 1/3 Vin, or 3.33V. When this happens, the 555 output goes high, and the 555 waits for the threshold input ("th") to reach 2/3 Vin, or 6.67V. As the capacitor charges, the threshold input slowly rises until it reaches the required level. Then, the timing interval ends, the output goes low, and the capacitor is discharged.

When the capacitor is discharged enough so that the trigger reaches 3.33V, then a new timing interval begins. The end result is a square wave.

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