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.
This circuit was found in an American publication from 1969. Transistors are from the time and can be...
We found this circuit in a 1998 documentation. The circuit makes use of CMOS logic and must be...
This 1 transistor circuit is from a Radio Electronics from December 1958… The circuit is battery...