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 is from 1968 documentation. We can assemble it with more modern components such as a...
Found in a 90s Silicon Chip documentation this circuit is intended to protect a speaker system in...
The UVTRON tube is an ultraviolet radiation sensor. In this circuit from Radio Electronics from...