This simple audio signal tracer or follower can be made with a BC548 instead of the original transistor. The output is made in a small crystal phone. The power can be from 5 to 9V.
This circuit drive an LED with constant current determined by the values of the resistors of 10...
This transistorized oscillator was published in the July 1953 issue of Radio Electronics....
This circuit is from 1995 documentation. Integrated circuits are used which can still be found...