A cool gadget just to watch! This project is a 10 band, octave centered, audio spectrum analyzer with a 26 dB range, 2 speed display ballistics and input level control.
The left and right audio channels are buffered and summed by a three of the op-amp's in a quad package. The fourth op-amp is utilized as a buffer, driving the inputs to 10 analog band pass filters. Each band pass filter is octave centered and gain adjusted to compensate for filter characteristics. CD4066 analog switches connect each filters output to a precision rectifier. The rectifier output feeds a 10 step bar graph chip - LM3915. The display rows are driven by the LM3915, the columns switched by transistors driven by the logic selecting the CD4066 outputs.