555 timer.
Astable mode. Fill any three of R1, R2, C, f — solve the fourth, plus duty cycle and timing.
The 555 astable, worked through
In astable mode a 555 charges its timing capacitor C through R1 and R2 in series, then discharges it through R2 alone, flipping the output each time the capacitor crosses ⅔ and ⅓ of the supply. The result is a continuous square wave whose timing depends only on R1, R2 and C.
t_HIGH = ln(2) · (R1 + R2) · C, t_LOW = ln(2) · R2 · C, and
f = 1 / (ln(2) · C · (R1 + 2·R2)), which is the familiar
f ≈ 1.44 / ((R1 + 2·R2) · C).
Example. R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF.
t_HIGH = 0.693 × 11,000 Ω × 0.00001 F ≈ 76.2 ms and
t_LOW ≈ 69.3 ms, so the period is about 145.6 ms, the frequency about
6.87 Hz and the duty cycle 52.4%: a comfortable LED blink.
Gotcha: the duty cycle can't go below 50%. In this basic circuit the
capacitor charges through both resistors but discharges through only one, so
duty = (R1 + R2) / (R1 + 2·R2) is always above 50%. Shrinking R1 gets you
close, but R1 can't be zero: it's what limits current into the discharge pin. For a duty
cycle under 50%, put a diode across R2 (the formulas above no longer apply) or invert the
output. And budget for capacitor tolerance: electrolytics are often ±20%, and the same
circuit with a real 12 µF runs at 5.72 Hz, not 6.87 Hz.
Questions people actually ask
Does this calculator do monostable mode?
No, it covers astable mode only. For a monostable (one-shot) 555 the output pulse lasts t = 1.1 × R × C, so 100 kΩ with 10 µF gives a pulse of about 1.1 seconds.
Why can't I get a 50% duty cycle from a 555?
In the basic astable circuit the capacitor charges through R1 + R2 but discharges through R2 only, so the high time is always longer than the low time. Making R1 much smaller than R2 gets close to 50%; going below 50% needs a diode across R2 or an inverter on the output.
How do I pick parts for a 1 Hz blink?
Fix the parts you have and solve for the one you don't. With R1 = 1 kΩ, C = 10 µF and f = 1 Hz, leave R2 empty and the calculator returns about 71.6 kΩ. The nearest standard value, 68 kΩ, gives about 1.05 Hz.
Why is my 555 running at a different frequency than calculated?
Usually component tolerance. Electrolytic capacitors are often ±20% and resistors ±5%, and large electrolytics also leak. With very small capacitors, stray capacitance from a breadboard adds to C. Measure the real capacitor or trim R2 to tune.
Does the supply voltage change the 555 frequency?
Ideally no, because the ⅓ and ⅔ switching thresholds scale with the supply. In practice the frequency shifts slightly with voltage. The classic bipolar NE555 needs roughly 4.5 V or more; CMOS versions such as the TLC555 run from lower supplies.