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.

Voltage controlled oscillator with the ICM7556 (CB22369E)
The integrated circuit used is a dual 555 CMOS. The circuit is from an English documentation and...
OR gate LM3900 CIR6569S CB15637E (CB15637E)
A 3-input OR gate can be made with an LM3900 amplifier, as shown in the figure. The purpose of the...
Bench Top Power Supply (CB17396E)
We found this conventional power source with a regulating transistor in a 2002 documentation. The...