Practice code oscillator (CB21090E)

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

Digital control with relays (CB23773E)
With the coded activation of the relays, it is possible to control which of the 16 outputs...
Programmable Current Shield (CB19955E)
The integrated circuit used in this application is not very common. Transistors and resistors...
Mains Voltage Indicator (CB11532E)
This simple circuit makes use of neon lamps to indicate the voltage of a power grid. The lamps are...