555 Timer — Astable
R1, R2, C → frequency, duty cycle, and high/low times for the classic astable hookup.
Your recent runs (stored only in your browser)
No calculations yet — results land here so you can compare runs.
The engineering
The capacitor shuttles between ⅓ and ⅔ VCC forever: charging through R1+R2, discharging through R2 alone. That asymmetry is why the plain astable can't reach 50% duty — the high time always wins. Make R2 ≫ R1 to approach 50%, or hang a diode across R2 to charge through R1 only.
Keep R1 above about 1 kΩ so the discharge transistor isn't asked to sink VCC through a dead short, and remember the timing capacitor's leakage and dielectric matter at long periods — film or C0G beats electrolytic when the math has to hold.
Where this math comes from
Hans Camenzind designed the 555 in 1971 as a contractor for Signetics, sketching it at home after the company's own marketing had rated the idea unpromising. His trick was replacing a programmable-timer scheme with two comparators, a flip-flop, and a three-resistor divider — the '555' echoing those 5 kΩ resistors, by legend if not by his own telling.
Released in 1972, it became the best-selling IC in history — estimated at a billion units a year by the 2000s — because it made 'time' a component: two resistors and a capacitor, and the ratiometric divider cancels supply-voltage drift for free.
- 1919Henri Abraham & Eugène BlochThe multivibrator — two-state RC timing circuits.
- 1971Hans CamenzindDesigns the 555 under contract to Signetics.
- 1972SigneticsNE555 released — timing becomes a three-part bill of materials.
See the full timeline of the math behind every calculator →
Runs entirely in your browser — nothing you enter leaves this page. Your recent runs are stored only on your device.