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.

3-Bit AD Converter (CB9027E)
This fast converter using operational was found in a 70s American documentation. The circuit can be...
Crossing Signal For Railroad Modeling 3 (CB3856E)
This circuit uses two sensors that are placed at the intersection of a miniature railroad, causing the LED...
RF Power Meter (CB4885E)
This RF wattmeter was found in an American 1980s documentation. The components are common and the...