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.
We found this circuit in a 1976 documentation, but it can still be easily assembled using BC548...
This circuit of an old publication produces the insect-like noise that drives them away. The...
This circuit aims to increase the selectivity of receivers whose FI is 455 kHz. The circuit is...