Electronics · Free Calculator
555 Timer Monostable Calculator
Find the output pulse width of a 555 one shot (monostable) timer, or the resistor for a target time.
t = 1.1 RCOne shotDelay timerResistor for time
Pulse width1.099 s
How this result was calculated
- t = 1.1 × R × C = 1.1 × 100 kΩ × 10 µF = 1.099 s
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555 Monostable Formula
t = ln(3) × R × C ≈ 1.1 × R × C
The timer starts on a falling edge at the trigger pin (pin 2) and the output stays high for time t.
Worked example
R = 100 kΩ and C = 10 µF give t = 1.1 × 100,000 × 0.00001 = 1.1 s.
Key insight: electrolytic capacitors can be 20 % off and leak, so for accurate long delays use a film capacitor or a counter IC instead of a very large C.
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Frequently Asked Questions
What is the formula for a 555 monostable?
t = 1.1 × R × C.
How long a delay can a 555 make?
Minutes are possible, but leakage in large electrolytic capacitors makes long delays inaccurate.
Why is it 1.1 RC?
The capacitor charges from 0 to 2/3 of VCC, which takes ln(3) = 1.0986 time constants.
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