555 Timer Astable Mode Calculator
Calculate frequency, duty cycle, and resistor/capacitor values for a 555 timer in astable (oscillator) mode.
Ideal for electronics projects and prototyping.
The 555 timer IC is one of the most widely used integrated circuits ever made. In astable mode, it operates as a free-running oscillator — producing a continuous square wave output without any external trigger.
The circuit uses two resistors (R1 and R2) and one capacitor (C) to set the frequency and duty cycle.
Key formulas for 555 astable mode:
Charge time (output HIGH): t1 = 0.693 × (R1 + R2) × C
Discharge time (output LOW): t2 = 0.693 × R2 × C
Total period: T = t1 + t2 = 0.693 × (R1 + 2×R2) × C
Frequency: f = 1/T = 1.44 / ((R1 + 2×R2) × C)
Duty cycle (% time HIGH): D = (R1 + R2) / (R1 + 2×R2) × 100%
Note: In the standard 555 astable circuit, the duty cycle cannot go below 50% because the capacitor always charges through R1+R2 but only discharges through R2. To achieve a 50% duty cycle, set R1 to a very small value (but not zero — always use at least 1 kΩ to protect the IC). To achieve below 50% duty cycle, add a diode bypass around R2.
Typical component values:
- R1 and R2: 1 kΩ to 10 MΩ (1 kΩ minimum for pin 7 protection)
- C: 100 pF to 1000 µF (larger = lower frequency)
Frequency ranges achievable:
- 1 Hz (slow blink): R1=1kΩ, R2=72kΩ, C=10µF
- 1 kHz (audio tone): R1=1kΩ, R2=71.5kΩ, C=0.01µF
- 100 kHz (fast PWM): R1=1kΩ, R2=6.2kΩ, C=1nF
The 555 timer IC runs on 5–15V supply. Output current is up to 200 mA (sinking or sourcing). The NE555 is bipolar; the CMOS TLC555 or LMC555 is better for battery-powered circuits (lower supply current).
Supply voltage does not affect frequency — only R1, R2, and C determine timing. This makes 555 circuits very reliable for timing applications.