Practice code oscillator (CB21090E)

Found in Popular Electronics from the 90's, this circuit consists of an audio oscillator for Morse code practice.

High Voltage Regulator (CB14323E)
The circuit shown was found in the National Semiconductor's Linear Applications Handbook of 1994....
Telephne Audio Interface (CB21269E)
This circuit from a Popular Electronics shows how to take the audio signal from a telephone line...
Capacitive discharge ignition (CB24182E)
This circuit is from a Radio Electronics from April 1972. The circuit uses power transistors of...