What triggers the monostable?

A negative-going pulse on pin 2 (trigger) that drops below 1/3 Vcc starts the timing cycle.

Can I retrigger during the pulse?

Standard 555 is not retriggerable. The pulse runs its full duration regardless of new triggers.

How do I make longer delays?

Increase R or C. For very long delays (minutes) use large capacitors or cascade two timers.

Monostable 555 Timer Calculator

Calculate the output pulse width for a 555 timer in monostable (one-shot) mode.

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Timing Resistor

kΩ

Timing Capacitor

µF

Pulse Width

1.100 s

Pulse Width (ms)1,100.0 ms

Pulse Width vs Timing Resistor

View formula & how it works →

555 Timer Monostable Mode

Formula

T = 1.1 x R x C

A trigger pulse starts timing. The output goes high for time T then returns low. The circuit requires a new trigger for each pulse.

Example Calculation

R = 100k, C = 10 uF.

01T = 1.1 x 100000 x 0.00001
02T = 1.1 seconds
03T = 1100 ms

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