Found in old American documentation, this circuit is intended for the control of heating elements. The SCR must have current according to the controlled load. Transistors are from the time.
The circuit shown was found in the National Semiconductor's Linear Applications Handbook of 1994....
This circuit from a Popular Electronics shows how to take the audio signal from a telephone line...
This circuit is from a Radio Electronics from April 1972. The circuit uses power transistors of...