555 Timer Calculator
Calculate the frequency, period, and duty cycle of a 555 timer in astable mode from R1, R2, and capacitor values.
Also covers monostable pulse timing.
The 555 timer IC is one of the most widely used integrated circuits in electronics history, introduced by Signetics in 1972. In astable mode (free-running oscillator), it produces a continuous square wave whose frequency and duty cycle are set by two resistors and a capacitor.
The key formulas: Frequency (Hz): f = 1.44 / ((R1 + 2×R2) × C) Period (seconds): T = 1 / f = (R1 + 2×R2) × C / 1.44 High time (seconds): t_high = 0.693 × (R1 + R2) × C Low time (seconds): t_low = 0.693 × R2 × C Duty cycle (%): D = (R1 + R2) / (R1 + 2×R2) × 100
Where:
- R1, R2 = resistance in Ohms (Ω)
- C = capacitance in Farads (F)
Worked example: R1 = 10,000 Ω (10 kΩ), R2 = 47,000 Ω (47 kΩ), C = 0.1 μF (0.0000001 F)
f = 1.44 / ((10,000 + 2×47,000) × 0.0000001) = 1.44 / (104,000 × 0.0000001) = 1.44 / 0.0104 = 138.5 Hz
Duty cycle = (10,000 + 47,000) / (10,000 + 2×47,000) × 100 = 57,000 / 104,000 × 100 = 54.8%
Component value guidelines:
- R1: 1 kΩ – 1 MΩ (must not be zero — short circuit risk)
- R2: 1 kΩ – 10 MΩ
- C: 100 pF – 1000 μF (larger = lower frequency)
Frequency ranges by application:
- LED blinker: 0.5–2 Hz (C = 10–100 μF)
- Tone generator: 200–4,000 Hz (C = 10–100 nF)
- PWM motor control: 1–20 kHz (C = 1–10 nF)
- RF (not ideal for 555): above 500 kHz — use dedicated oscillator ICs
50% duty cycle trick: Place a diode across R2 (cathode toward pin 7). This makes t_high ≈ t_low ≈ 0.693 × R1 × C, achieving near-perfect 50%.