In this 2001 circuit we have the way to trigger an SCR with a phototransistor. The circuit appears elsewhere in this section, obtained from publications from different eras.
Found in 90s Popular Electronics, this circuit fires a load through a MOSFET from an audio signal...
Found in Radio Electronics from the 90s, this circuit uses components from that time. However, it...
This automatic volume control for audio signals was found in American documentation from the 90s....