This circuit was found in an Electronic Engineering of the 80s. The circuit indicates when a telephone line is being used, being useful in case of extensions.
Found in Radio Electronics from the 90s, this circuit generates signals in the 60 Hz to 1.4 Hz...
This circuit was obtained from an internet documentation. The LED is inverted in the original, and the...
This simple test for general purpose NPN and PNP transistors was found in a 1974 publication....