This squelch circuit used in transmitters was found in a magazine for radio amateurs 73 in the 1980s. The circuit admits equivalent components.
Using SCRs, this circuit from a 1967 General Electric manual allows full wave load control with SCRs....
This interesting circuit from a publication from the 1970s, measures the intensity of the sound...
This simple NPN and PNP transistor tester configuration can use the low current range of a common...