The integrated circuit used in this 2002 project is not very common. It is powered directly by the telephone line, requiring no power supply.
Found in documentation from the 1980s, this circuit can be used with other operational ones. The...
This circuit was published in the December 1987 Radio Electronics magazine. It uses two 555s to...
This circuit identifies the polarity of DC signals and if the signal is AC. The circuit is from...