Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
This circuit, which uses only common transistors, was obtained from a 1988 publication, but can be...
The integrated circuit used in this filter is not very common today, but the configuration can be implemented...
Published in Radio Electronics magazine in March 1963, this circuit uses 4 transistors of the...