This circuit is from Electronics Now magazine, February 1987. The number of channels is high depending only on the distances of the stations and the losses in the cables.
The circuit in the figure can be used to trigger a triac or even a comparator in a project that...
Suggested by a 90s Electronic Design, this circuit drains only 0.25 mA from its source. Listening...
Found in a 1960 Sylvania manual, this circuit can be implemented with a modern equivalent...