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.

Temperature Compensating Amplifier Stage (CB4247E)
In the figure we have a simple amplifier stage with a temperature compensated general purpose...
FET and Bipolar Amplifier (CB18870E)
This good power amplifier was found in a Japanese magazine from the 1970s. The circuit uses...
Complete Alarm System (CB24383E)
This circuit was found in an article in Radio Electronics magazine from May 1975. The circuit has...