Found in an old Popular Electronics this circuit is still current. The SCR can be of the 106 series and the relay according to the voltage used in the supply. The voltage can be 12 V.
This squelch is activated by its own noise, acting automatically. It is used in communications...
This circuit for a simple alarm system was suggested in an article in Electronics Now magazine. The...
Found in American documentation from the 70s, this circuit uses components of the time, uncommon...