This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and transistor a BC558. The circuit is for the 110V power grid.
Other voltage comparators can be used in this circuit that we found in a Popular Electronics from...
Published in a February 1978 Radio Electronics magazine article, this digital circuit uses some of...
This circuit was obtained in an English magazine from 1980, but it can be easily assembled today....