Found in the documentation on PCs, this circuit excites LEDs whose current is determined by R3.
This circuit tests crystals up to a few megahertz with ease. The circuit is from a 1977...
We found this circuit in a 1983 English magazine, but it can be easily assembled with the BC548...
In fact, this is the circuit also known as “vox”, that is, a relay triggered by sound. This...
