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555 Timer Astable Mode

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In astable mode, the NE555 timer generates a continuous square wave output without any external trigger. This calculator computes the oscillation frequency, total time period, HIGH time, LOW time, and duty cycle based on your resistor and capacitor selections.

f = 1.44 ÷ ((R1 + 2×R2) × C) · Duty Cycle = (R1+R2) ÷ (R1+2×R2) × 100%
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How to use this calculator

Step-by-Step Instructions

1. **Enter R1** This is the resistor between VCC (pin 8) and the discharge pin (pin 7). Typical range: 1kΩ to 1MΩ.
2. **Enter R2** This is the resistor between pin 7 (discharge) and pin 6/2 (threshold/trigger). R2 controls the LOW time.
3. **Enter C** This is the timing capacitor connected between pin 6/2 and ground. Typical range: 1nF to 1000µF.
4. **Click Calculate** Results show frequency (Hz), total period, HIGH time, LOW time, and duty cycle percentage.

Worked Example 1Hz LED Flasher

R1 = 6.8kΩ, R2 = 68kΩ, C = 10µF f = 1.44 / ((6800 + 136000) × 0.00001) = **1.01 Hz**

This creates an LED that blinks approximately once per second with ~52.4% duty cycle.

Pro Tips

  • In standard astable mode, the duty cycle is always above 50%. For a 50% duty cycle, add a diode across R2 or use R1 much smaller than R2.
  • Use a 10nF decoupling capacitor on pin 5 (Control Voltage) to ground for stable operation.
  • For audio frequencies (20Hz–20kHz), use capacitors in the 1nF–10µF range with resistors in the 1kΩ–100kΩ range.