This circuit is from National Semiconductor using an LM11, which is uncommon nowadays. The circuit can be implemented with other operational ones.
The function generator XR2206 is not very easy to obtain, but if you get one you can assemble this...
This 1990 circuit can also be used with fixed sound systems. What it does is trigger a set of LEDs...
In the figure we have a sensor circuit that makes use of common transistors and a photo diode as...