555 Timer Calculator — Astable & Monostable Frequency

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

R1	R2	C	Frequency	Duty cycle
1 kΩ	10 kΩ	100 nF	686 Hz	52%
10 kΩ	10 kΩ	100 nF	480 Hz	67%
10 kΩ	100 kΩ	1 μF	6.86 Hz	52%
1 kΩ	1 kΩ	10 nF	48 kHz	67%
10 kΩ	10 kΩ	10 μF	4.8 Hz	67%

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

Mode

Ra (kΩ)

Rb (kΩ, astable only)

Capacitor C (µF)

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.

⏱️ 555 Timer Calculator

555 timer — Quick answer

⏱️ 555 Timer Calculator

Formula

Frequently Asked Questions

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