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.

Automatic Lighting Control (CB15752E)
The original purpose of the figure circuit, suggested by National Semiconductor, is to keep the...
Active Cut-Out Amplifier (CB15422E)
The amplifier shown in the figure was obtained from the Anlog Devices linear integrated circuits...
Alarm (CB196E)
When the sensor X1 is opened, the relay is energized. When waiting, the current drained is very...