555 Timer Calculator
Pick the parts and watch the output: frequency, duty cycle and pulse length update as you type.
Presets:
Waveform
Green: output on pin 3. Orange dashes: the timing capacitor charging between โ and โ of the supply.
About this tool
Astable formulas
High time = 0.693 ร (R1 + R2) ร C, low time = 0.693 ร R2 ร C, so f โ 1.44 รท ((R1 + 2ยทR2) ร C). The capacitor charges through R1 and R2 but discharges through R2 only, which is why the duty cycle is always above 50% in this circuit. Make R2 much bigger than R1 to get close to 50%, or add a diode across R2 to go below it.
Monostable
A falling edge on pin 2 starts a single pulse of 1.1 ร R ร C seconds. Keep the trigger pulse shorter than the output pulse.
Real-world accuracy
Expect a few percent difference from the calculation: electrolytic capacitors are often ยฑ20%. Use film capacitors and 1% resistors for accurate timing, and the CMOS versions (TLC555, LMC555) for low power or frequencies above about 100 kHz.