At many discos
one might very well find a light organ
. Look for a bunch of colored lights that are blinking to the music
. That is most likely a light organ.
The light organ is a circuit with several channels or outputs. These outputs are activated when certain frequencies or a range of frequencies are hit. With a four channel light organ one could imagine the four ranges being bass, low mid, high mid, and treble. In a techno song with a hard bass line, the bass output would be blinking according to the bass line.
One can easily purchase a light organ, but that's much to easy. For about $50 one can purchase parts from Radio Shack, or any electronics store. A search for 3 or 4 channel light organs will yield a variety of schematics. Finally, a few hours of soldering and you can have your very own dance party in your very own room.