Practice code oscillator (CB21090E)

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

Coded Lock with Alarm (CB17377E)
We found this circuit in a year 2000 documentation. It must be powered by a voltage of 5 V as it...
LM381 Magnetic Head Amplifier (CB25281E)
This circuit was found in the February 1990 issue of Radio Electronics magazine. The circuit uses...
Voltage Comparator Shield (CB4588E)
Voltage comparators, such as those in the LM139/239/339 series, consist of devices that can be...