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.

Signal injector (CB22158E)
This circuit was found in an English publication from 1989. The integrated circuit is an...
MW Experimental Radio (CB4517E)
This circuit was obtained from a 1964 magazine and has some interesting features such as the input...
Gain 10 Amplifier (CB25636E)
Using a general-purpose transistor this AC amplifier was found in the November 1993 issue of...