How 555 monostable timer works
A short low trigger sets the output high and releases the timing capacitor to charge through R1. When C1 reaches two-thirds of the supply, the output goes low and C1 discharges. The trigger must return high before the pulse ends; a held-low trigger is outside this one-shot calculation.
Ideal thresholds give ln(3)RC, commonly rounded to 1.1RC. The simulated supply rises over 1 µs, then one short trigger starts the pulse after the configured delay. This is a conventional non-retriggerable timing example, not a guaranteed retriggerable one-shot.
Change the form inputs or click a component value on the schematic. Choose Try in CircuitSim to simulate the configured circuit.