555 Timer — Monostable
One-shot pulse width from R and C: t = 1.1·R·C.
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The engineering
Trigger below ⅓ VCC and the output snaps high while C charges from zero toward VCC; the ride to the ⅔ threshold takes exactly ln(3) time constants — the '1.1RC' every databook rounds to. One trigger, one clean pulse: switch debouncing, missing-pulse detection, staircase delays.
Two field cautions: hold the trigger low longer than the pulse and the output stays stretched (the trigger is level-sensitive), and beyond a few minutes of pulse width, capacitor leakage current competes with the charging current through a multi-megohm R — a CMOS 555 (ICM7555) buys you another decade.
Where this math comes from
The one-shot predates the chip — Abraham and Bloch's 1919 multivibrator family included the monostable, and vacuum-tube one-shots timed everything from radar range gates to pinball scoring before silicon shrank them.
Camenzind's 1971 design at Signetics folded the astable and monostable into one 8-pin part, and the monostable hookup became the canonical 'my first timing circuit': one resistor, one capacitor, one predictable pulse.
- 1919Henri Abraham & Eugène BlochMultivibrator circuits, including the one-shot.
- 1971Hans Camenzind555 designed at Signetics — monostable mode built in.
- 1972SigneticsNE555 ships; 1.1·R·C enters every databook.
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