In the post #38 circuit, when the lamp is working there is about 130mV rms across the piezo. Is that likely to produce an annoying background sound?

To answer your question I used circuit below and made sound box, using medicine jar, to increase output.
Threshold of sensitivity for me was about 10 cm (from sound box hole to ear).
I heard soft 50 Hz sound.
For younger people threshold was up to 50 cm.
They heard crisp sound.

Sound box:

 

Thanks for doing a practical test.

 

If you could accept an optical warning instead of an audible one, then just a standard 6V 6W bulb in series with the heat lamp might suffice. The bulb would be under-run at only 0.75A so should have a long life.

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The TS already has an optical warning of a mains incandescent light failure.

The light bulb is dark!

Oh! Remotely...

Nevermind

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