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.

Transistor test (CB22545E)
16 commercial test circuit for tubes and transistors appeared in Radio Electronics magazine in...
Quinary Trigger (CB6805E)
Suggested by an American publication, this circuit has 5 operating conditions indicated in the...
True RMS Meter (CB24789E)
This circuit with an integrated circuit of the time was found in the magazine Radio Electronics of April...