Can I get a 50% duty cycle from a 555 timer?
Not from the standard two-resistor astable — but a single diode fixes it, and opens up any duty cycle, not just 50%. Put your target frequency and duty cycle in and get the resistors to buy. Everything runs in this page; nothing is uploaded.
Design the diode-mod astable
Pick the capacitor you have, the frequency and duty cycle you want — any value from just above 0% to just below 100% — and get the exact R1/R2, the nearest standard (E24) values, and what those buyable resistors really produce.
Common questions
Can I get a 50% duty cycle from a 555 timer?
Not from the standard two-resistor astable — it charges through R1 + R2 and discharges through R2 alone, so the high time is always longer than the low time and 50% is a limit it only approaches, never reaches. Add a diode across R2, forward-biased so it shorts R2 out of the charge path, and the capacitor charges through R1 alone instead. High time and low time no longer share a resistor, so any duty cycle is possible, including 50% or below.
What resistor values give an exact 50% duty cycle with the diode mod?
R1 = R2. With the diode in place, high time is ln(2) times R1 times C and low time is ln(2) times R2 times C, so they are equal exactly when the resistors are equal — and the frequency formula collapses to the familiar single-resistor form, f = 1 divided by (2 times ln(2) times R times C).
Does the diode need to be a specific type?
Any small switching diode works electrically — a 1N4148 is the usual choice. What varies is its forward voltage drop while conducting: roughly 0.6 to 0.7 V for silicon, lower for a Schottky part. That drop is not in the formula below, so it is a real source of error, worse at low supply voltages or a small R1 where the drop is a bigger fraction of the charging voltage. A Schottky diode shrinks that error; it does not change which duty cycles are reachable.
Can the diode mod reach duty cycles below 50%, not just down to it?
Yes. Duty cycle is R1 divided by (R1 + R2) once the diode is in place, and that ratio is free to land anywhere between 0% and 100% by choosing R1 smaller or larger than R2 — the 50%-or-above restriction was specific to the plain two-resistor astable, and the diode removes it rather than just extending it to exactly 50%.