mcd. LEDs used in this design are in that range, and all have a
maximum current rating of about 22 mA. The LEDs are pulsed
and are never turned “on” for longer than 50 msec. This keeps
current down to around 15 mA and allows you to parallel
diodes without the need for current-limiting resistors.
You may want to include resistors while experimenting,
and they should be added between the outputs of the
Darlington transistors and the cathode side of the LEDs. Be
sure to use the LINE OUT from the audio source and not the
amplified output. There is a minimum voltage threshold for the
input (approx 1V peak to peak). It’s best to start at the
maximum LINE OUT setting from the source device and then
lower it as needed for personal taste.
Building the Display Board
Figure 2 is an example layout of LEDs for the
CHROMATICON. The board used for the display is a 4” x 6”
plated through hole prototyping board. With 15 I/O lines
available, there are many options for the organization of LEDs.
If you connect more than one LED per line, connect the
additional lights in parallel. The individual and grouped diodes
are wired to female SIP headers of eight pins each on either
side of the board, mounted in the center as shown in Figure 2.
The only pin that matters for alignment to the driver circuit
is the +5V supply line. The remaining connections to the LEDs
do not need to follow any strict order and should be made
opportunistically trying to avoid using jumper wires.
Building the Driver Board
Figure 3 is a photo of the driver circuit that was used in
the prototype. The driver contains all of the electronics needed
for the circuit. The components on this board were connected
together using silver conductive ink, but any interconnect
method will work. Figure 4 is the back or interconnect side of
the driver board. The connections for the prototype were
made with conductive ink but any method of connection can
be used. The parts are very close together, making it easier for
Figure 5 shows the reflected inking pattern for the design.
The connections from the back need to be reflected to insure
that the pin 1 locations for the ICs are correctly connected. This
same pattern can alternatively be used for soldering wire busses
to connect the components. The routes are color coded to
separate power, ground, and signals. Figure 6 shows a side view
of the board with the correct placement of the male headers.
They need to be mounted so the pins are facing the direction
shown to plug into the female headers on the display board.
Wiring for the circuit is brought to the base of the
document holder behind the display board. This consists of the
programming cable, audio input, and power supply for the
circuit as shown in Figure 6. You may want to use a shielded
36 November 2013
■ FIGURE 3. Component side of the driver circuit.
■ FIGURE 4. The wiring side of the driver board.
■ FIGURE 5. Connection side of the driver board.
■ FIGURE 2. Interconnect for the LEDs and a
close-up of the display wiring.