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.

DC to DC SMPS (CB9669E)
This circuit was found in an American documentation from the 80's. The components are from that...
Optical Power Modulator (CB14676E)
The circuit shown is from the Texas Instruments optoelectronics manual. It is intended for audio...
LED drive I (CB18890E)
This circuit drive an LED with constant current determined by the values of the resistors of 10...