This 40-second delay switch was found in a 1965 Electronic Design. The unijunction transistor might be the 2N2646 and the SCR a sensitive modern equivalent.
Found in a 90s Popular Electronics, this circuit triggers a load from the audio signal applied to...
This old receiver (1967) uses germanium transistors from the time and coupling transformers,...
I found this circuit in a Popular Electronics from 1992. The circuit can be easily implemented with BC549...