555 Timer Calculator

How it is calculated

For a 555 timer wired as an astable oscillator, the tool computes f = 1.44 / ((R1 + 2·R2) × C) and duty = (R1 + R2) / (R1 + 2·R2) × 100. R1 and R2 are the two timing resistors in ohms, C is the timing capacitor in farads and f is the output frequency in hertz. There is no unit conversion, so a 1 µF capacitor must be typed as 1e-6 (or 0.000001). The duty cycle is the share of each period the output is high, and because the capacitor charges through R1 + R2 but discharges through R2 alone, it is always above 50% and approaches 100% only when R1 is much larger than R2. All three values must be above zero. The constant 1.44 is an approximation of 1/ln 2, and real parts vary with tolerance.

Common mistakes

• Entering the capacitor in microfarads. Typing 1 for a 1 µF capacitor means 1 farad, and the tool returns 0.00 Hz. Use 1e-6.
• Entering resistances in kilohms. The fields are in ohms, so a 10 kΩ resistor is 10000, not 10; the frequency would be off by a factor of a thousand.
• Expecting a 50% duty cycle. In this basic circuit the duty cycle is always above 50%, so a pure square wave needs a different arrangement, such as a diode across R2 or a divide-by-two stage.
• Applying the result to monostable or bistable circuits. The formula is only for the astable configuration with two resistors and one capacitor.
• Treating the frequency as exact. The 1.44 factor is rounded and real resistors and capacitors have tolerances, so the actual frequency can differ by several percent.