555 Timer Calculator

Calculate the frequency and duty cycle of a 555 astable oscillator, design one for a target frequency, or find a monostable pulse width.

If the resistors fall outside 1 kΩ – 1 MΩ, choose a different capacitor and calculate again.

t = 1.1 × R × C. The output goes high for this long after each trigger.

How the 555 timer works

The 555 is one of the most popular integrated circuits ever made. Inside it are two comparators, a flip-flop and a discharge transistor. A timing capacitor charges through resistors, and the comparators switch the output when the capacitor voltage reaches two-thirds and one-third of the supply voltage. Because both thresholds are fractions of the supply, the timing hardly changes when the supply voltage changes.

Astable mode formulas

In astable mode the 555 runs continuously and produces a square wave. The capacitor charges through R1 and R2 and discharges through R2 only.

• Time high: t high = 0.693 × (R1 + R2) × C
• Time low: t low = 0.693 × R2 × C
• Frequency: f = 1 ÷ (0.693 × (R1 + 2R2) × C), about 1.44 ÷ ((R1 + 2R2) × C)
• Duty cycle: (R1 + R2) ÷ (R1 + 2R2) × 100%

Example: R1 = 1 kΩ, R2 = 10 kΩ and C = 1 µF give a frequency of about 68.7 Hz and a duty cycle of 52.4%. The number 0.693 is the natural logarithm of 2, which this calculator uses exactly.

Monostable mode

In monostable mode the 555 waits for a trigger pulse, then holds its output high for a fixed time t = 1.1 × R × C before returning low. It is used for timers, switch debouncing and delay circuits. For example, R = 100 kΩ and C = 10 µF give a pulse of 1.1 seconds.

Frequently Asked Questions

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