This circuit is used in transceivers, enabling input selection by diode switching through polarity reversal. The circuit is from an old American documentation on telecommunications.
We find this circuit in an English 80's documentation. The circuit can be made with more modern...
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
This is one of many Morse code audio oscillators that can be found in this section. The circuit...