Practice code oscillator (CB21696E)

In the figure we have an astable multivibrator driving a speaker to produce audio signals for Morse code practice.

Preamplifier and Equalizer (CB18860E)
This circuit is from a 1968 publication. It uses germanium transistors from the time, having...
The Transistor as Switch (CB922E)
This circuit can be used as shield or to drive powerful loads from low current switches such as...
CBS Columbia TR 260 Radio (CB18446E)
This is a 6-transistor transistor receiver manufactured in the 60s. The circuit is a...