Free · Physics models
Ideal 555 Oscillator and One-Shot Timing
Calculate ideal astable period, duty cycle and monostable pulse time from resistances and capacitance.
- Formula & worked example
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
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Formula and method
time low = 0.69315; time high = 1.3863. This worked example uses the displayed units and declared assumptions.
time_low = ln(2) * R2_a * C_a; time_high = ln(2) * (R1_a + R2_a) * C_a; f = 1 / (ln(2) * (R1_a + 2 * R2_a) * C_a); T = time_low + time_high; duty = 100 * (ln(2) * (R1_a + R2_a) * C_a)/((ln(2) * R2_a * C_a) + (ln(2) * (R1_a + R2_a) * C_a)); T_mono = ln(3) * R_mono * C_mono
Worked example
Enter these known values and leave the other values blank.
- Mode select
- 1
- R1
- 1000 Ω
- R2
- 1000 Ω
- C
- 0.000001 F
- Time high
- 1.3863 ms
- Time low
- 0.6931 ms
- Cycle duration (T)
- 2.0794 ms
- Cycle frequency (f)
- 480.9 Hz
- Duty cycle
- 66.67 %
Assumptions and limitations
- The standard 555 threshold model uses ln(2) for astable charging/discharging and ln(3) for monostable charging. Component tolerances, leakage, supply limits and output loading are omitted. This astable topology has duty cycle above 50 percent.
- Clear an unknown to solve a supported inverse; keep known values fixed with locks. Singular or underdetermined inverses may remain incomplete.
Common questions
What assumptions apply?
The standard 555 threshold model uses ln(2) for astable charging/discharging and ln(3) for monostable charging. Component tolerances, leakage, supply limits and output loading are omitted. This astable topology has duty cycle above 50 percent.
Which unknowns can I solve?
Editable controls preserve the supported captured inverse branches. Clear the desired unknown and provide enough independent known quantities; singular or underdetermined inverses may remain incomplete.
References
The standard 555 threshold model uses ln(2) for astable charging/discharging and ln(3) for monostable charging. Component tolerances, leakage, supply limits and output loading are omitted. This astable topology has duty cycle above 50 percent. Clear an unknown to solve a supported inverse; keep known values fixed with locks. Singular or underdetermined inverses may remain incomplete.