Practice code oscillator (CB21090E)

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

Silicon diode tester (CB23829E)
We found this test circuit in Radio Electronics magazine from July 1962. The circuit uses...
Photoelectric Relay (CB12887E)
We found this circuit in an old Argentine publication from 1973. It makes use of a large LDR (2.5...
Inductance Bridge (CB4799E)
In the figure we have an inductance bridge for values between 1 and 30 uH at a frequency of 5 MHz....