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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Electronic Micro Organ (CB4440E)
This circuit, which uses only common transistors, was obtained from a 1988 publication, but can be...
Multi-Pass Filter with Multiple Feedback MC1776 (CB4499E)
The integrated circuit used in this filter is not very common today, but the configuration can be implemented...
Two-Channel Mixer (CB23869E)
Published in Radio Electronics magazine in March 1963, this circuit uses 4 transistors of the...