LED CUBE


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Multiplexing
How to control 64 LEDs without using 64 individual wires? Multiplexing!
Running a wire to the anode of each led would obviously be impractical, and would look really bad.
One way to get around this, is to split the cube into 4 layers of 16x16 LEDs.
All the LEDs aligned in a vertical column share a common anode (+).
All the LEDs on a horizontal layer share a common cathode (-).
Now if i want to light up the LED in the upper left corner in the back (0,0,3), I just supply GND (-) to the upper layer, and VCC (+) to the column in the left corner.
If i only want to light up one led at a time, or only light up more than one layer at the same time.. this works fine.
However, if I also want to light up the bottom right corner in the front (3,3,0), I run into problems. When I supply GND to the lower layer and VCC to the front left column, I
also light up the upper right led in the front (3,3,3), and the lower left LED in the back (0,0,0). This ghosting effect is impossible to workaround without adding 64 individual
wires.
The way to work around it is to only light up one layer at a time, but do it so fast that the eye doesn't recognize that only one layer is lit at any time. This relies on a
phenomenon called Persistence of vision .



Making the cube, template
Soldering grids of 4x4 LEDs freehand would look terrible!
To get 4 perfect 4x4 grids of LEDs, we use a template to hold the them in place.
I wanted to make the cube as easy as possible to make, so I chose to use the LEDs own legs as much as possible. The distance between the lines in the grid was
decided by the length of the LED legs. I found that 25mm (about an inch) was the optimal distance between each led (between the center of each led that is!) to enable
soldering without adding or cutting wire.
Find a piece of wood large enough to make a 4x4 grid of 2,5cm on.
Draw up a 4x4 grid of lines.
Make dents in all the intersects with a center punch.
Find a drill bit that makes holes small enough so that the led will stay firmly in place, and big enough so that the led can easily be pulled out (without bending the
wires..).
Drill the 16 holes.
Your ledcube template is done.


Making the cube, connecting the layers
Now that we have those 4 layers, all we have to do is to solder them together.
Put one layer back in the template. This will be the top layer, so choose the prettiest one
Put another layer on top, and align one of the corners exactly 25mm (or whatever distance you used in your grid) above the first layer. This is the distance between the
cathode wires.
Hold the corner in place with a helping hand and solder the corner anode of the first layer to the corner anode of the second layer. Do this for all the corners.
Check if the layers are perfectly aligned in all dimensions. If not bend a little to adjust. Or re-solder of it's the height distance that's off. When they are perfectly aligned,
solder the remaining 12 anodes together.


Choosing resistor values
There are two things to keep in mind when choosing a resistor value for your leds.
1) The LEDs
2) The AVR
The AVR has a maximum combined current rating of 200 mA.
This gives us 12mA to work with per LED.
You also don't want to exceed the maximum current your leds are rated to.
I used 220 ohm resistors on my cube. This gave me about 12mA per led.
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