In use telephone indicator (CB8657E)

Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be used with extensions.

Instrumentation Amplifier (CB20984E)
Found in National Semiconductor publication from the 1980s. This gain-100 circuit can be...
Audiability Control (CB12063E)
We found this configuration in a 1975 publication. It can be implemented easily, as it uses only...
6 Transistors Radio (CB16303E)
The high number of transistors on this radio for the medium wave range is an indication of its...