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Relay Buzzer

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Recipe #35 — relay buzzer

A relay can even make itself oscillate. Put a normally closed contact in series with its own coil and the relay repeatedly energizes, interrupts itself, releases, and starts again.

Feedback loop: the normally closed contact powers the coil, but energizing the coil opens that very contact. Current stops, the relay releases, the contact closes again, and the cycle repeats.

An oscillator with moving metal

This is an oscillator even though there is no capacitor setting the period. Mechanical travel, magnetic build-up, spring force, contact bounce, and coil current determine the repetition rate.

Another way to think of it

It is a machine that continually turns itself off, then turns itself back on because turning off removes the force that held it off.

Contacts wear

Rapid opening and closing causes arcing and mechanical wear. This circuit is excellent for understanding feedback, but solid-state oscillators are usually better when long life, quiet operation, or precise frequency is required.

Try these yourself

Why does the relay repeatedly click? Its normally closed contact interrupts its own coil current The battery reverses polarity The coil becomes a capacitor The contact is permanently welded

Energizing the coil opens the series NC contact, which removes coil current and causes the relay to release again.

What kind of concept does this demonstrate? Feedback Rectification only Transformer turns ratio only Digital storage only

The relay output mechanically changes the circuit that controls its own input current.

Why is this not an ideal precision oscillator? Mechanical timing and contact behavior vary It has no moving parts It contains a quartz crystal Its frequency is fixed by pi exactly

Mechanical travel, spring force, magnetic response, and contact bounce make its timing variable and imprecise.

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