m5 Timer Calculator — Astable & Monostable Mode">

⏱️ 555 Timer Calculator

Calculate frequency, duty cycle, and time delay for 555 timer circuits

Electrical Calculator

555 timer — Quick answer

The 555 timer IC generates square-wave oscillations (astable mode) or single timed pulses (monostable mode). Frequency and duty cycle are set by two resistors and one capacitor.

Astable: f = 1.44 / ((R1 + 2×R2) × C)  Hz
Duty cycle = (R1 + R2) / (R1 + 2×R2)
Monostable: t = 1.1 × R × C  seconds

  • R1, R2 — timing resistors in ohms (typically 1 kΩ to 1 MΩ)
  • C — timing capacitor in farads (typically 1 nF to 100 μF)
  • f — output frequency in Hz
  • t — output pulse width (monostable) in seconds

Worked example: R1 = 10 kΩ, R2 = 20 kΩ, C = 10 nF (astable): f = 1.44 / ((10k + 40k) × 10nF) = 1.44 / (5 × 10-4) = 2.88 kHz. Duty cycle = 30k / 50k = 60%.

Common 555 astable configurations

R1R2CFrequencyDuty cycle
1 kΩ10 kΩ100 nF686 Hz52%
10 kΩ10 kΩ100 nF480 Hz67%
10 kΩ100 kΩ1 μF6.86 Hz52%
1 kΩ1 kΩ10 nF48 kHz67%
10 kΩ10 kΩ10 μF4.8 Hz67%

Standard / source: Texas Instruments NE555 / LM555 datasheet; Bipolar 555 is the original 1972 design.

Used for: LED flashers, tone generators, time delays, PWM control, square-wave clocks, oscillators in educational circuits.

⏱️ 555 Timer Calculator

Formula

This calculator uses the standard 555 timer calculator formula:

Astable Frequency Formula
f = 1.44 / ((Ra + 2×Rb) × C)

Frequently Asked Questions

What is a 555 timer? ⌄

The 555 timer is a versatile integrated circuit used to generate precise time delays and oscillations. It operates in three modes: astable, monostable, and bistable.

What is astable mode in 555 timer? ⌄

In astable mode, the 555 timer continuously oscillates, producing a square wave output. Frequency = 1.44 / ((Ra + 2×Rb) × C).

What is monostable mode in 555 timer? ⌄

In monostable mode, the 555 timer produces one output pulse of defined width when triggered. Time delay = 1.1 × Ra × C.

How do I calculate the duty cycle of a 555 timer? ⌄

Duty cycle = (Ra + Rb) / (Ra + 2×Rb) × 100%. To get 50% duty cycle, use a diode in parallel with Rb (so charging goes through Ra only).

How do I change the frequency of a 555 timer? ⌄

Frequency f = 1.44 / ((Ra + 2Rb) × C). Increase C or R values to lower frequency; decrease them to increase frequency. Frequency is inversely proportional to both.

What capacitor value gives 1 Hz with a 555 timer? ⌄

For 1 Hz: 1 = 1.44/((Ra+2Rb)×C). With Ra=Rb=47kΩ: C = 1.44/((47k+94k)×1) = 1.44/(141,000) ≈ 10.2 µF. Use a 10 µF capacitor with fine-tune resistors.

Need Advanced Calculations?

Use the full AI Calculator suite — 350+ precision engineering and science calculators with PDF report export.

⏱️ Open Full Electrical Calculator — Free

Related Calculators

Ohm's Law · LED Resistor · Resistor Color Code · Voltage Divider · RC Time Constant · Op-Amp Gain · BJT Transistor · all electrical calculators

Standards & method

✓ Independently verified 12 July 2026
Basis
First principles
Method
Standard 555 astable and monostable timing relations, per the classic device datasheet. Real timing varies with supply voltage and temperature.
Core formula
Astable: f = 1.44 / ((R1 + 2·R2)·C)  ·  Monostable: T = 1.1 · R · C
Why this matters
The astable duty cycle cannot go below 50% in the classic configuration — R1 is in the charge path but not the discharge path. You need a diode across R2 to get below it.
Independently verified
12 July 2026 — Formula re-derived from first principles and verified numerically against hand-computed reference cases, including edge cases and unit handling.

Results are for guidance. Verify against the current edition of the governing standard and have a qualified professional review before use in practice.