This is a simple sawtooth oscillator using a 555 timer chip.

A timing interval starts when the trigger input ("tr") goes lower than 1/3 V_in, or 3.33V. When this happens, the 555 output goes high, and the 555 waits for the threshold input ("th") to reach 2/3 V_in, or 6.67V. A PNP transistor acts as a current source to steadily charge the capacitor. The threshold input slowly rises until it reaches the required level. Then, the timing interval ends, and the capacitor is quickly discharged through the "dis" input.

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

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