This circuit uses an optocoupler with a phototriac or optotriac to drive an external triac. The circuit is from a Radio Electronics magazine from December 1989.
This VFO was found in a 1980s documentation. The central frequency is given by the crystal and the...
This is one of the many possible siren configurations for the 555 integrated circuit. We found it in...
The circuit shown here is a CCO. The frequency here depends on the current through the...