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555 Timer One-Shot

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Recipe #27 — 555 timer one-shot

The 555 timer packages an RC threshold detector, latch, and discharge switch into one inexpensive IC. The simplest useful connection is a one-shot: one trigger produces one timed output pulse.

Trigger: a brief low pulse on pin 2 starts the timing interval. The 555 output goes high while C1 charges through the timing resistance. When C1 reaches the internal threshold, the output returns low and the discharge transistor resets C1.

T ≈ 1.1 RC

Why the 555 is convenient

The previous RC timer needed an external threshold circuit and reset method. The 555 contains comparators, a latch, and a discharge transistor internally, so the same timing job becomes much easier to wire.

Monostable means one stable state

The circuit normally rests with its output low. A trigger temporarily pushes it into the other state, but after the RC interval it automatically returns to its stable state.

For an actual build, tie RESET pin 4 to +9 V when it is not being used, and follow the device datasheet for supply decoupling. Pin 5 CONTROL is commonly bypassed to ground with about 0.01 µF when appropriate.

Try these yourself

What starts the one-shot timing interval? A trigger pulse on pin 2 Removing the ground symbol Shorting the LED Changing the package color

A short trigger pulse causes the 555 to begin one timing interval.

What mainly sets the pulse duration? The timing resistance and C1 Only the LED resistor Only the battery size The number printed on the IC

For a standard 555 monostable, pulse time is approximately 1.1RC.

What does monostable mean? One stable resting state No stable states Two power supplies One transistor only

The circuit has one normal resting state and returns to it after each triggered pulse.

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