Just in case you haven't been following my columns here on EE Times -- augmented by videos on YouTube -- I'm building a 3D 8x8x8 RGB LED cube, which was inspired by this article written by our very own Max Maxfield.
Previously, I discussed the testing of 1,000 RGB LEDs and the straightening of my tinned copper wire. In this column, I will cover how I actually set about constructing the 8x8 panels for my cube. This will include more detail on the jigs I created to help me with this task. But before we start, first take a look at this video.
Let's quickly establish the main criteria for my cube, so you can see where I'm coming from.
- The LEDs will be of the 5mm RGB common anode variety.
- The LEDs will be pitched one inch apart in the X, Y, Z planes.
- The LEDs will be interconnected using 20swg tinned copper wire.
- Either the wire or the LED leads will be formed into loops to provide more robust solder joints.
- The construction will be as square and as straight as possible.
- The cube will be made using eight 8x8 panels mounted vertically.
- The eight panels will be soldered to a base PCB. (The image below shows two panels temporarily mounted on a prototype PCB.)
- The LED anodes will be connected horizontally in eight planes numbered 0 to 7 from bottom to top.
- Cathode connections will be vertical (24 per panel: eight red, eight green, and eight blue).
- The front of the cube will be the flat face of an 8x8 panel with the horizontal anode rails facing forward.
- The cube will be mounted centrally on the base PCB.
- The PCB base will secure the cube and provide connection to other PCBs.
- The top surface of the PCB will be clear of any solder joints other than the cube tails passing through it.
- Any other connections to the PCB will be via surface mounted connectors on the underside.
- Any control circuitry will be on separate PCBs.
The reasoning behind my choices
I decided to use 5mm LEDs because 12mm LEDs look out of proportion on a one-inch pitch. Smaller LEDs would have looked OK, but I favor the 5mm variety. I think the one-inch pitch keeps the overall construction in proportion with the 5mm LEDs. Also, the final result will be quite sturdy. A larger pitch would make the cube more flimsy, and a smaller pitch would make it look too cluttered.
My choice to use 20swg wire (instead of the 22swg I've seen used by others) was for better rigidity due to the additional wire diameter. Since the wire is twisted to improve its straightness (as discussed in my most recent column), its diameter is reduced slightly anyway. Working with straight wire eases construction and improves the cube's overall aesthetics.
The LEDs' leads will be formed into loops, and the copper wire will pass through those loops before soldering. Wire-to-wire joints will be similar. This should improve connection reliability with the drawback that replacing any LEDs that fail will be harder. I will reduce the risk of LED failures by testing them regularly throughout the construction process. Also, before the final cube assembly, I will carry out a burn-in test on all the LEDs at the 8x8 panel stage. I think 24 hours per panel should be enough to highlight any weak components.
I have no desire to solder hundreds of interconnecting wires from the cube tails to the control circuits, as I have seen others do. I know that this is, in effect, a prototype, but I really want to build this cube only once. I have the utmost confidence that it will work fine, so I will build it as if it were the final item. With this in mind, the cube will be mounted directly on a printed circuit board (PCB). Also, I want that board's upper surface -- the part visible to the observer -- to be clear of any solder joints other than the cube tails passing through it. The only other holes will be a few vias. All other connections and fixings will be on the PCB's underside.
I have considered surface mounting the control circuit components on the the main board's underside. However, I have decided to develop the control circuits on separate PCBs, thereby allowing me to modify the circuit design if I have a change of heart. This way, I won't have to remanufacture my cube, and the whole system will become much more modular.
The base PCB will have a white solder mask and black silk screen. Also, it will be manufactured professionally to present the best possible appearance -- an appearance I would never be able to achieve myself. But this comes at a significant cost. (The board design will be discussed further in a future column.)
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