555 Timer Monostable Pulse Generator

This circuit uses a 555 Timer as a monostable one-shot. A 0 V to 5 V clock drives TRIG with a 20 ms period and 90% high time, so each short low-going interval starts one output pulse.

Suggested Applications

• Pulse stretching for mechanical buttons, sensors, or short trigger events.
• One-shot timing intervals for indicators, alarms, and simple control circuits.
• Comparing a narrow trigger interval against the wider pulse generated by the timer.

Featured Component

The 555 Timer combines threshold comparators, an internal latch, an output driver, and a discharge transistor. In monostable operation, the trigger sets the latch and the timing capacitor resets it after the selected interval.

Design Context

RT is 100 kOhm and CT is 100 nF, giving a nominal pulse width of about 1.1 x RT x CT, or roughly 11 ms. The +5 V rail feeds VCC, RESET, and RT. THRESH and DISCH share the timing node, CT returns that node to ground, and CCTRL bypasses the control-voltage pin. The output drives a current-limited LED so the pulse is visible as a load event as well as a waveform.

The Vtrig probe shows the low-going trigger clock, Vtim shows the timing capacitor ramp, and Vout shows the generated output pulse.

Expected Behavior

When Vtrig drops below the trigger threshold, Vout should switch high and the LED load should turn on. While Vout is high, Vtim charges upward through RT. When Vtim reaches the internal threshold, the discharge transistor pulls the timing node low and Vout returns low. With the current values, each output pulse should be about 11 ms wide, and the 80 ms transient run shows several trigger-and-pulse cycles.

Suggested Experiments

Change RT or CT to adjust the output pulse width, compare Vtrig to Vout, or reduce the clock period to see what happens when new trigger events arrive before the one-shot has fully reset.