555 timer calculator
Frequency, duty cycle, and HIGH/LOW times for the astable 555 — from Ra, Rb, and C.
Astable (free-running) mode. Keep Ra ≥ 1k so pin 7 isn't asked to short the supply, and C ≥ 100 pF so strays don't dominate.
The circuit this computes
555 Timer (astable) — fully explained →How it works
The 555 bounces its capacitor between ⅓ and ⅔ of VCC, forever. Charge runs through Ra + Rb (output HIGH), discharge through Rb alone into pin 7 (output LOW). One RC time constant per half, ln(2) each: f ≈ 1.44 / ((Ra + 2Rb) · C).
Common questions
The capacitor charges through Ra + Rb but discharges through Rb only, so the HIGH half is always longer. Get close to 50% by making Rb much larger than Ra — or hang a diode across Rb so charge bypasses it, the classic mod.
Keep Ra ≥ 1k (pin 7 shorts Ra to ground every cycle — too small cooks the chip), both resistors under ~10M (leakage territory), and C ≥ 100 pF (below that, strays and chip capacitance dominate and the math drifts).
Capacitor tolerance, mostly — ceramics are ±10–20% and electrolytics worse. The resistor formula is honest; the cap rarely is. For frequencies that matter, use a film cap (±5%) or trim with Rb.