Step 6: Wire up the 8 LED Strands

Picture of Wire up the 8 LED Strands
I chose wire wrap as the wiring method for this project because:
  1. it’s cheap
  2. there are no toxic fumes
  3. there is no risk of overheating and destroying LED’s
  4. it’s easy to undo, so repairs are easier
  5. it’s relatively quick to do
  6. it’s as reliable as soldering – perhaps more reliable, since there’s no risk of cold-soldered joints
  7. the wire is so thin that it can be threaded into the gaps between the foam matrix pieces
If you are not familiar with wire wrapping, Wikipedia has a detailed introduction to the technique.  I was a bit worried that the 30AWG wire would be too thin to handle the current running through the matrix.  I consulted some tables on the Internet, I found that 30AWG solid wire has a resistance of 105.2 Ω/1000 ft. I estimated an upper bound of 4 ft of wire, so that’s ~.4Ω – probably insignificant for this application. Next, I found that the current carrying ability (Ampacity) is estimated at .144A by most tables, but one actually had it listed at .86A. The LED’s I used run at a maximum of 20mA per color. In a column, a maximum of 1 LED at a time is lit, so it’s definitely in spec. On the other hand, if a whole row is driven w/ the with all 3 colors lit, we can end up with 8 x 3 x 20mA = 480mA. This is way over the .144A spec, but below the .86A spec. I decided to just try it, and if I encountered problems, I would put multiple wires in parallel for the rows.  After testing, I found that using single wires for the rows worked fine ... they did not heat up at all.

The schematic diagram for a common anode 8x8 RGB matrix is depicted above. We are going to wire our matrix, composed of 5mm LED's, in the same fashion.

We need a total of 8 strands of 8 LED's.  Each pair of LED's is connected with three 4" pieces of wire. The anode is the lead which is slightly longer.  In my photo above, the pins, from left to right, are Blue/Green/Anode/Red.

The red wires connect the red channel,  the white wires the green channel, and the blue wires the blue channel.  4" may sound a bit long, since the cells are only 2" wide, but I when you account for stripping the wire on both ends, and leaving enough slack to manipulate the LED's in while assembling the grid, 4" is a good length.  You will need 49 4" pieces of wire for each color - 147 pieces total.  While working on this step, I discovered a few things which make for a quicker and cleaner job:
  1. orient the pins of your LED's the same way while wiring them (e.g. blue pin on the left, red pin on the right)
  2. connect the R/G/B wires between each pair of LED's before moving to the next LED.  At first, I was connecting all the reds, then all the blues, then all the greens.  This method was a bit messier.
  3. spread the pins apart a bit before wrapping.  Otherwise, the wrapping tool will get caught on the adjacent wraps as you try to insert it 
  4. start the first wrap about halfway into each lead
  5. when connecting the second wire on each lead, just stack the wrap against the previous wrap.
Note that the anode pin is left unconnected at this stage.  This is because the anodes form the rows, which are perpendicular to the R/G/B cathode wires, which form the columns.
spiker5463 years ago
where did you connect the anode wires?
aridee879923 years ago
Hi Lincomatic!

I'm nearly finished with this project, but I was wondering if you put current limiting resistors anywhere in this circuit?

lincomatic (author)  aridee879923 years ago
You don't need current limiting resistors. The current is adjusted via software