electronics.tools Electronics design calculators

555 Timer Calculator

Uses exact ln(2), not 0.693. With R1 = 10 kΩ, R2 = 47 kΩ and 10 nF, f is 1.387 kHz.

By Jasper · Updated

Calculator

PASS1.387 kHzduty 54.81 % · exact ln(2)/ln(3)

Uses exact ln(2) and ln(3), not the 0.693 / 1.44 / 1.1 datasheet roundings. Standard astable duty is always > 50 %.

f
1.387 kHz
Duty
54.81 %
tH
395.09 µs
tL
325.78 µs
  • Pass Astable: f = 1387.21 Hz, duty = 54.81 %

Formula

Astable high time

tH=ln2⋅(R1+R2)C tH = ln(2)·(R1+R2)C

Astable frequency

f=1ln2⋅(R1+2R2)C f = 1 / (ln(2)·(R1+2 R2)C)

Monostable pulse

T=ln3⋅RC T = ln(3)·R C (1.1 RC is a labelled approximation)

Variables

Symbol Meaning Unit Typical range Precision Default
mode Mode — — 1 sig astable
R1 Astable RA Ω ≥ 0 mΩ R1 cannot be 0 4 sig 10 kΩ
R2 Astable RB Ω ≥ 0 mΩ 4 sig 47 kΩ
C Timing capacitor F ≥ 0 pF C below ~100 pF meets parasitics 4 sig 10 nF
R Monostable timing resistor Ω ≥ 0 mΩ 4 sig 100 kΩ
f Frequency Hz — 4 sig —
D Duty cycle % — 4 sig —

Worked example

Astable 10 kΩ / 47 kΩ / 10 nF

Input Value
mode · Mode astable
R1 · Astable RA 10 kΩ
R2 · Astable RB 47 kΩ
C · Timing capacitor 10 nF
Output Value
f · Frequency 1.387 kHz
D · Duty cycle 54.81 %

Astable 1 kΩ / 10 kΩ / 10 µF

Input Value
mode · Mode astable
R1 · Astable RA 1 kΩ
R2 · Astable RB 10 kΩ
C · Timing capacitor 10 µF
Output Value
f · Frequency 6.87 Hz
D · Duty cycle 52.38 %

How it works

The bipolar 555 is still the fastest way to get a square wave or a one-shot on a bench. The datasheet roundings 0.693, 1.44 and 1.1 are convenient; this page keeps the exact logarithms so reverse astable (f and duty → R1, R2) round-trips.

Choose astable, monostable or reverse astable. R1 cannot be zero. For RC settling tables without the 555 thresholds, use the RC time constant calculator.

Pitfalls

  • R1 = 0 shorts the discharge pin to VCC on the standard astable.
  • Standard astable duty is always > 50 %. Duty ≤ 50 % needs a diode across R2, which this page does not model.
  • C below about 100 pF meets pin and board parasitics that the ideal formula ignores.

FAQ

Why not 1.44 / ((R1+2 R2)C)?

1.44 is 1/ln(2) rounded. This calculator uses exact ln(2) and ln(3).

Can I get 50 % duty with the standard circuit?

No. Duty = (R1+R2)/(R1+2 R2) is always above 50 % while R1 > 0.

What is the 1.1 RC figure?

A labelled approximation of ln(3) ≈ 1.0986. The page reports both.

Does this cover the CMOS 555?

Timing formulas are the same; threshold currents differ. Use the datasheet for the part on the board.

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