DIY LED Array Text Display Marquee

Uploaded by nerdkits on 19.12.2008

Hi. In this video we are going to show you how with a simple microcontroller, a piece
of cardboard, and a whole bunch of LEDs, you can put together a do it yourself LED marquee
just like this one. Now it's fairly easy to turn an LED on or
off with one of our NerdKits microcontroller kits.
However, even if they were possible to use every single pin on your NerdKits microcontroller
as an output pin, we only have 23 pins, which is only enough to direct or control 23 LEDs.
That's not enough for an impressive display. In order to get our 5 by 24 display, we have
to use a few tricks. Since we need to have the NerdKit connected
to a computer, and some of the pins are taken by the oscillator and power, we only have
17 pins we can use as output pins. For our first trick we use something called
the row-column addressing to address the LEDs. Each free pin of the microcontroller is hooked
up to either a row or a column wire. The LEDs themselves are hooked up with one pin connected
to a row wire and one pin connected to a column wire. Using the microcontroller, we cycle
through each one of the row wires turning them on one at a time. For that row and every
LED we want to turn on, we turn the corresponding column wire low. This drives current through
the LED, turning it on. For every LED we want off, we turned that column high. Using this
method, we can individually control each one of the LEDs by cycling through the row wires
every couple of milliseconds. We mention before that we have 17 pins available.
This means that using row column addressing we can address 5 rows and up to 12 columns
for a total of 60 LEDs. Now that just wasn't enough for our R&D team here at NerdKits,
so they figured out a neat trick to double the amount of columns.
LEDs are diodes. One of the properties of diodes is that they only conduct current in
one direction. If you apply a reverse voltage, it just won't turn on. This allows us to hook
up 2 LEDs to every column and every row. Now, for every row wire, we first turn on the appropriate
forward LEDs, all the LEDs drawing current from the row wire to the column wire, and
then we switch and turn on all the appropriate backward LEDs, all the LEDs drawing current
from the column wire to the row wire. Now, instead of cycling through all the rows every
couple of milliseconds, we also switch the polarity so we can get to all the LEDs. This
doubles the numbers of columns we have to a 5 by 24 LED display, all on one NerdKit.
Looking closely at the wiring, on the front, you have the row wires going all the way across,
and on the back, you see that every 2 LED columns are connected to one column wire.
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