Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
You can program the 4018 in other ways, as shown in the figure. In this 7 divider configuration, for...
This circuit was suggested by Siliconix in a 1980s publication. The circuit uses components from the time...
This circuit, for frequencies up to 100 kHz, was found in American documentation from 1970. The...