This fast Schmitt trigger operates at frequencies from 10 Hz to 100 Hz, with very short rise time. The circuit is from an American documentation from 1968.
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...