tweak, its performance fell short of what must have been
too great an expectation on my part.
Just like other similar circuits I have built in the past,
I find that the wholesale summing of frequencies in this
manner to be somewhat uninteresting. Personalizing these
circuits typically means just selecting a different color
combination of lights for the three bands, and scalability is
limited to the strength and number of lights. This is where
the CHROMATICON and the color organ circuits part
With color organ circuits, the patterns that can be
displayed are fixed by the physical placement of the LEDs.
With the CHROMATICON, however, displayed patterns
are controlled by software. Instead of a sluggish response
to the music input, the CHROMATICON comes to life
with each note played, and every song displays in some
unique way. It has caused me to have a renewed interest
in my collection of music, and I am anxious to see how
each song will get displayed.
The following is a simple overview of the
fundamentals of how music is composed, and may help in
understanding the approach I have taken toward my
technique of sound to light conversion. Traditional music is
comprised of individual frequencies that make up the seven
note or diatonic scale consisting of the notes C, D, E, F, G,
A, and B. If you include the sharps (notes raised a half
tone), there are actually 12 notes to the chromatic scale
written as C, C#, D, D#, E, F, F#, G, G#, A, A#, and B.
On a piano, these 12 note scales
repeat as the first octave (CDEFGAB)
to seventh octave (CDEFGAB), plus a
minor third (A0 to C8) across a
keyboard of 88 keys. An octave is the
simplest interval in music. It is the
interval between notes in one octave
to the same note in a lower or higher
octave. The range of frequency
between notes from one octave to
the next is either one half or twice the
frequency of the note.
For example, the A note from the
fourth octave has a frequency of 440
Hz. The A note one octave higher
(A5) is at 880 Hz, and the A note an
octave lower (A3) is at 220 Hz. With
the octave being such a fundamental
part of creating music, it just makes
sense to me to build a circuit that can
visualize the notes played in the full
range of octaves used to compose
Digital Hardware for
an Analog Task
The heart of this circuit is the PICAXE, but it can be
constructed using any processor that is capable of
frequency measurement. The PIC, PICAXE, and Stamp
micros are able to measure frequency with the COUNT or
PULSIN command. I haven’t tested the circuit with the
Atmel family (Arduino), but I believe that it does have a
similar Pulse(IN) command.
The operation of the circuit is extremely
straightforward. A line level output from an audio source is
coupled to an input pin on the 08M2 by way of a 0.1 µF
capacitor through a limiting resistor. The PIC 20M2
chosen for this project allows for up to 15 outputs to drive
the LEDs, and one pin necessary for the audio input.
There are two design camps when using this method
to convert sound to light. In one camp, by hard-wiring
patterns of LEDs more patterns can be had using fewer
I/O ports. An alternative method is to individualize the
connections to the LEDs. This method will exhaust I/O
lines faster, but allows for greater flexibility in displaying
patterns of LEDs by simply changing the software. This is
how the circuit covered here was designed.
Following the schematic shown in Figure 1, a left or
right channel line out from an audio source is coupled to
the C. 6 pin of the PIC using a 1 µF capacitor. The only
difference between this circuit and the minimal circuit is
the use of an additional Darlington array to meet the
higher I/O count.
Any medium output LED will work for the display. The
medium output range is usually between 2,500 and 8,000
November 2013 35
■ FIGURE 1. Example system design using the
PIC 20M2 micro-controller.