555 Timer Astable Circuit Calculator

555 Timer Astable Calculator & Waveform

Capacitor (C)

Resistor 1 (R_A)

Resistor 2 (R_B)

Charge Time (T_H)

Discharge Time (T_L)

Total Period (T)

Frequency (f)

Duty Cycle (%)

Output Waveform

Duty Cycle: --%

Total Period: --

Use our 555 Timer Astable Mode Calculator to quickly calculate frequency, duty cycle, and time period based on resistor and capacitor values.

What is Astable Mode?

In astable mode, the 555 timer keeps turning ON and OFF again and again, making a square wave. It needs two resistors (RA and RB) and a capacitor (C). No trigger input is required — it starts working on its own when powered.

How It Works

The capacitor charges through both R1 and R2.
When the voltage across the capacitor reaches 2/3 of the supply voltage, the output goes LOW.
The capacitor discharges through R2.
When the voltage drops to 1/3 of the supply voltage, the output goes HIGH again.
This repeats in a loop, making a square wave signal.

Applications

LED blinkers
Tone generators
Clock pulses
PWM circuits
Simple delay circuits
Flashing lights

How to Use 555 Timer Astable Circuit Calculator

Enter the capacitor value and select its unit (nF, µF, etc).
Enter resistor values RA and RB and choose units (Ω, kΩ, MΩ).
Click “Calculate” to get the results.
See the timing and waveform details.

Output Parameters

TH (High Time) – How long the output is HIGH
TL (Low Time) – How long the output is LOW
T (Total Period) – One complete cycle
f (Frequency) – How many cycles per second
Duty Cycle – Percentage of time output is HIGH

Formulas Used in 555 Timer Astable Circuit Calculator

These are the formulas used for the calculation:

$T_H = 0.693 \times (RA + RB) \times C$
$T_L = 0.693 \times RB \times C$
$T = T_H + T_L = 0.693 \times (RA + 2 \times RB) \times C$
$f = \frac{1.44}{(RA + 2 \times RB) \times C}$
$\text{Duty Cycle} = \left( \frac{T_H}{T} \right) \times 100\%$

Example Calculation

Given: RA = 1kΩ, RB = 100kΩ, C = 10µF

$T_H = 0.693 \times (1k + 100k) \times 10\mu F = 0.693 \times 101000 \times 10^{-6} = 0.699 \text{ sec}$
$T_L = 0.693 \times 100k \times 10\mu F = 0.693 \times 100000 \times 10^{-6} = 0.693 \text{ sec}$
$T = T_H + T_L = 0.699 + 0.693 = 1.392 \text{ sec}$
$f = \frac{1}{1.392} \approx 0.718 \text{ Hz}$
$\text{Duty Cycle} = \left( \frac{0.699}{1.392} \right) \times 100 \approx 50.2\%$

Component Selection Guide

Frequency	RA	RB	C	Use
0.1–1 Hz	10k–100kΩ	100k–1MΩ	10–100µF	Slow LED blinkers
1–10 Hz	10k–47kΩ	47k–100kΩ	1–10µF	Warning lights
10–100 Hz	1k–10kΩ	10k–47kΩ	0.1–1µF	Audio tones
100–1000 Hz	1k–4.7kΩ	4.7k–10kΩ	0.01–0.1µF	Alarms
1–10 kHz	470Ω–1kΩ	1k–4.7kΩ	0.001–0.01µF	High-pitched tones
10–100 kHz	100–470Ω	470Ω–1kΩ	100–1000pF	Max 555 frequency

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