This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
The presented circuit monitors a line indicating if it is busy if there is an extension. The...
July 2017 - The Texas Instruments step-down converter module of the same name IC can now be...
This circuit uses the LT6106 from Linear Technology, from where we took it from a 2005 manual....