Suggested by Raytheon in 1967 documentation this circuit uses a lightbulb and LDR optical coupler of the time. The transistor determines the maximum current.
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...