This simple step of low power audio is from a 1975 documentation. The transistor can be a BC548, and the transformer of the type found in small transistor radios and recorders of the time.
We found this circuit in a publication from the 1970s. The circuit has a logarithmic gain...
This simple low-power circuit uses, in the original version of 1965, germanium transistors. The...
This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The...