Practice code oscillator (CB21090E)

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

Astable with the Operational LM3900 (CB4511E)
Other series amplifiers such as the MC3301, MC3401 and LM2900 can be used on this circuit. We...
Electronic Alarm Clock (CB18726E)
The purpose of this circuit is to trigger an oscillator with the light of dawn. The tone of the...
Fast Inverter Amplifier (CB20146E)
This fast inverter amplifier with high input impedance uses operational amplifiers from the 80's,...