the octave to light all four lights, it creates a very
interesting effect. It helps to assign names for the ports
that is indicative of their physical locations. This same
technique can be used to indicate when notes from the
other octaves are played.
Using LEDs of the same color arranged in various
patterns creates another interesting effect. I find that by
using enough LEDs to make square, diagona,l or cross
patterns can produce the best effects with the least
number of LEDs. The layout of LEDs shown in Figure 2 is
an example of this suggested arrangement. The software
that is available for download was written expressly for the
arrangement of diodes shown in Figure 5. The code has
been well documented and should serve as a starting
point for a layout of your own design. This design can be
scaled up using several methods. One method to free up
I/O lines would be to hard wire the LED patterns. This
would allow you to have more patterns. Another method
would be to connect more LEDs in parallel to the existing
I/O lines. Each ULN2803 is rated up to 500 mA.
The duration for how long the LED is kept open
during operation is controlled by the value held in the
DELAY variable. In the program listing provided, the DELAY
is set to 50 milliseconds, but you may want to use
different delay times for the ranges of the discriminated
frequencies. There can be a noticeable difference using
longer on times for higher frequencies, and shorter times
for the lower frequencies.
Final Thoughts
This circuit is a single channel or mono design, so the
choice of using the left or right channel is up to you. A
stereo circuit may require more than just another input
due to the time needed for both sampling and processing
the results. A twin circuit would be ideal to produce a
stereo effect. A sensitive microphone can be used as an
input to the processor, as long as an amplifier is included
to boost the signal. An alternative to using a separate
board for the driver would be to mount the components
directly to the back side of the display board. The 4” x 6”
board for this project is a plated through hole design
which would allow the parts to mount on either side. If
you prefer that kind of design, feel free to email me if you
need help with that implementation.
However you decide to implement this circuit, I hope
you enjoy how it enhances the music you listen to as
much as I have. NV
November 2013 39
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