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.
This simple electroscope with bipolar transistors is not the most sensitive, but can detect static...
This oscillator for various frequencies was obtained from a 1980s telecom documentation. The...
This circuit is from a July 1970 Radio Electronics magazine. The sensors are metal probes. The 18 V...