Found in a 1992 book, this circuit is intended for operation with audio signals. Equivalent operational amplifiers can be used. The optical coupler is an unusual type today.
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...