This circuit is a monostable multivibrator, or one-shot, made with a 555 timer chip. Click the logic input on the left (the "H"), and the output goes high for a short time, and then it goes low again.
A timing interval starts when the trigger input ("tr") is brought low. When this happens, the 555 output goes high. This causes the capacitor to be charged until it reaches 6.67V. Then, the timing interval ends, the output goes low, and the capacitor is discharged through the "dis" input.
The capacitor in front of the trigger input causes the monostable to be negative-edge triggered. If the capacitor is replaced with a wire, and the logic input is held low too long, then the 555's output will start to oscillate.
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