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 for experimental radios and other applications is from a documentation from the 50's....
This circuit was found in a Popular Electronics from the 90s. The circuit uses general purpose...
This 20 Hz high-pass filter was obtained in a 1965 American publication. The operational amplifier must...