This high-gain circuit was found in an American publication from the 1990s. The circuit can use general purpose transistors and powered from a source or battery.
September 2016 - can be found at Mouser Electronics the new amplifiers with digital variable gain...
This receiver was found in an article in Electronics Now magazine in March 1994. The coils are...
This alarm circuit has a trigger delay system. The circuit was found in a Popular Electronics of the 90s....