Electronics / Circuits
555 Timer Calculator (Astable and Monostable)
Work out the frequency, duty cycle, and high and low times of a 555 timer as an oscillator, or the pulse length as a one-shot, from the resistor and capacitor values.
555 Timer Calculator (Astable and Monostable): In astable mode the capacitor charges through R1 and R2 and discharges through R2, so the output is high for 0.693 × (R1 + R2) × C and low for 0.693 × R2 × C, and f = 1.44 ÷ ((R1 + 2 × R2) × C). In monostable mode a trigger starts one pulse of 1.1 × R × C. With R1 = 1 kΩ, R2 = 10 kΩ, and C = 10 µF, the 555 blinks at about 6.86 Hz. Runs 100% locally in your browser with zero server file uploads.
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Astable: f = 1.44 ÷ ((R1 + 2·R2) × C); the output is high for 0.693 × (R1 + R2) × C and low for 0.693 × R2 × C, so the duty cycle is always above 50%; add a diode across R2 to go below. Monostable: one pulse of 1.1 × R × C after each trigger. Capacitor tolerance, often 10–20%, changes the timing by the same amount.
Choosing parts for a frequency
Pick a capacitor first, then the resistors: for about 1 kHz with C = 100 nF, R1 + 2 × R2 = 1.44 ÷ (1000 × 100 nF) = 14.4 kΩ; R1 = 1.5 kΩ and R2 = 6.8 kΩ give 15.1 kΩ, about 950 Hz.
To read the capacitor's printed code, use the capacitor code converter.
CMOS versions
Low-power CMOS versions such as the 7555 use the same formulas, run on lower voltages, and draw far less current, which suits battery projects.
The 556 is two 555 timers in one package.
How to use it
- Choose astable (a continuous square wave) or monostable (one pulse per trigger).
- Enter the resistors in kilohms and the capacitor in microfarads.
- Read the frequency, duty cycle, and timings, or the pulse length.
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Frequently asked questions
Why can the duty cycle not go below 50%?
The capacitor charges through both resistors and discharges through only R2, so the high time is always the longer one. A diode across R2 lets it charge through R1 alone, for duty cycles below 50%.
What values should I avoid?
Keep R1 above about 1 kΩ so the discharge pin is not overloaded, and the resistors below a few megohms; very small capacitors make stray capacitance matter.
How accurate is the timing?
As accurate as the capacitor, often only ±10 or ±20%; use film capacitors and 1% resistors for steadier timing.
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