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.

Unijunction Metronome (CB12281E)
This circuit is from a 1991 publication, but it can be assembled with more modern components. We...
Precision Current Source (CB14078E)
This circuit provides a current given by the components calculated according to the formula next...
Audio Booster (CB23540E)
Published in Radio Electronics in October 1963, this circuit increases the audio output of a small...