Power grid voltage failure indicator (CB7013E)

Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.

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Self-biasing JFET stage (CB25566E)
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Low Power Alarm (CB538E)
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VU-Meter (CB10589E)
This circuit was published in an English magazine from 1977, but it can be easily assembled since...