The human eye experiences a phenomenon known as
Persistence of Vision (POV). What this amounts to is that
above certain frequencies, the eyes retain an image.
Electronics have used this for years to save energy. Its
standard use is with seven-segment displays. You have
three digits with a seven-segment display (eight, if you use
the decimal). If you were going to use a gate for each
segment, you would need a microprocessor with gates of
3 x 7, or 21 gates. In this project, we have 59 LEDs so we
need 59 gates — a very large microprocessor indeed.
If we put eight LEDs anodes to eight gates and
connect all the cathodes to a switch, we can control the
eight LEDs to be on or off via the switch. Now, if you
connect another eight LEDs and connect their anodes to
the same gates, then add another switch to their cathodes,
we can control 16 LEDs by turning off and on their
cathodes and gates.
Continuing on for six more strings, we can control 64
LEDs with 16 gates: 16/2 = 8; 8x8 = 64. Another way to
look at it is we are using binary with two conditions:
either on or off — 28 = 64. The microprocessor used is
running at a rate of 4 MHz, so turning 64 LEDs on and off
is no problem. In fact, we have to add time delays to see
them pulse (or have that annoying flicker).
With this project, I decided to put a four-terminal LED
for the last LED and only use seven layers of eight LEDs, or
56 + 1 (three colored) = 57 LEDs. The remaining five
positions will remain empty. By strobing at a rate greater
than 60, the LEDs will seem to stay on to the eye. This is
taken care of in software, and saves power and
The printed circuit board (PCB) was designed using
ExpressPCB’s free software at www.expresspcb.com. The
board files are available for download at the article link,
along with the microprocessor programming files. A
complete kit is available from the Nuts & Volts Webstore.
All of the components go on the top of the board.
Solder IC1 socket and IC2 to the board. Note that the
pin 1 location is the square pad. Bend the leads of eight
December 2014 33
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A full size schematic
file is available at the