I have had many requests for this circuit. It was a very popular unit years ago. The basic idea of the project is to make different colored bulbs light at different frequencies of music. The circuit connects to the speaker outputs of your stereo or to the back of your speaker. The music passes through the transformer and the volume level is adjusted by the 5k ohm pot. Each light bulb is turn on by a different frequency of sound based on the resistor & capacitor combination in the gate circuit of the SCR. If the resistors R1, R2, or R3 are changed, the frequency of sound that will trigger the SCR will change. The isolation transformer is for protection.
From :: http://home.maine.rr.com/randylinscott/jan99.htm