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.

Simple photo sensor (CB25475E)
This simple circuit with LDR was found in the Radio Electronics magazine of July 1992. Any...
Bench Calibrator (CB19196E)
This circuit is tuned to produce pulses of 1.5 ms and 5.5 ms of separation for various settings with...
20 dB amplifier (CB8935E)
This circuit was obtained in a documentation from the 90's. Several common operational amplifiers...