This circuit, found in a 1958 magazine, is for reference only, since the components are not easy to find today. The transistors are germanium types and power supply is made by common cells.
This 1990 circuit can also be used with fixed sound systems. What it does is trigger a set of LEDs...
This interesting circuit of the American publication Electronics Handbook Volume XIII 1993 shows a...
Found in American documentation, this circuit can make use of modern equivalent transistors. The...