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555 Timer Metronome

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Recipe #28 — 555 timer metronome

Reconnect the 555 so it has no stable resting state and the one-shot becomes a repeating oscillator. At a few beats per second, the result is an electronic metronome.

555 ASTABLE OSCILLATOR — METRONOME 555 2 TRIG6 THRESH7 DISCH3 OUT1 GND8 VCC +9 V R1 10 kΩ P1 500 kΩ RATE C1 10 µF BEAT LED1 kΩ The capacitor repeatedly charges and discharges — the timer retriggers itself.
Astable mode: C1 repeatedly charges through R1 + P1 and discharges through P1 using the 555's internal discharge transistor. The output flips each time the capacitor reaches an internal threshold.
f ≈ 1.44 / ((R1 + 2R2)C)

Turn timing into rhythm

At slow rates the output can flash an LED like a metronome. Increasing the resistance or capacitance slows the beat; decreasing either one speeds it up.

The ordinary 555 astable is not exactly 50% duty cycle

The capacitor charges through R1 + R2 but discharges mainly through R2. That makes the HIGH and LOW times different. Later circuits can modify the charge/discharge paths when a more symmetrical waveform is needed.

555 wiring reminder: in a real build, tie RESET pin 4 high when unused, provide supply decoupling close to the IC, and treat pin 5 CONTROL as the datasheet recommends.

Try these yourself

What makes this 555 circuit astable?
The timing capacitor repeatedly crosses the internal thresholds, so the output keeps changing state without an external trigger.
What happens when P1 resistance is increased?
A larger resistance makes C1 charge and discharge more slowly.
Why are HIGH and LOW times usually unequal in the basic 555 astable?
C1 charges through R1 + R2 and discharges mainly through R2.
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