Practice code oscillator (CB21090E)

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

Low Side Current Sensor (CB20383E)
This current sensor employs the Texas Instruments LMV358A low voltage operational amplifier. This...
One Transistor Alarm (CB914E)
This one-transistor alarm can be used with much more sensors than the two shown by the figure....
SCR Bank for 1 kW Sources (CB20287E)
Found in an American publication from the 1980s, this circuit is designed to regulate the voltage of...