This circuit is from a Hands-On Electronics from the 90s. The integrated amplifiers used are well known. The range is a few tens of meters depending on the resistance of the cables.
We found this common application of the 741 operational amplifier in a 1977 documentation. The...
This circuit, found in an old 1967 publication, cuts the current from a source when the current...
This LED bar graph circuit was found in the October 1980 issue of Radio Electronics magazine. The...