August 25, 2014 AT 4:00 pm

24 x 24 LED Matrix Build

Thanks to Adam for sharing his project on Show and Tell! Check out more info on it here.

This project was briefly teased before, but it seemed like a good time for more details. Originally conceived as a coffee table build, it quickly morphed into what will eventually become a wall hanging and has been a test-bed for a lot of my LED work. Having worked a great deal with a variety of these digital LED strips, I noticed that in most cases they were manufactured in 0.5m sections and soldered together to form 5m strips. This is usually fine, but also means that the distance between pixels at the solder joint is 2-3mm shorter than the rest of the pixels. Being the type of person that wouldn’t be able to stop twitching over a few pixels in a matrix being misaligned from the rest, this just wouldn’t do. So the best solution was to make each row out of exactly one of these sub-sections so that all pixels are perfectly aligned. Not wanting to un-solder all of the joints to get at the 0.5m sections, I reached out to the manufacturer I typically order from in China about getting the strips in the raw sub-sections. Fortunately, they obliged. Armed with 24 half meter sections of LPD8806 strips, each with 24 pixels, I got to work laying out an evenly spaced grid on a sheet of acrylic.

After a couple laborious hours of laying out a perfect grid with a 24″ square and a lot of patience, I taped each of the LED strips with a rubber adhesive based strapping tape which I’ve found adheres extremely well to acrylic. Since it had to be wired as one continuous strip, the direction of each was alternated for every row. But first, power buses need to be created to provide the almost 34A (@5V) current the 576 pixels could draw. Originally, I wanted to design a 0.5m long PCB that would handle all of the power and signal routing. But I quickly figured out that I would need two separate designs (the signal pins are farther apart on one side than the other) and that a PCB that long, even a thin one, would cost nearly $75, per design, from OSHPark. $150 for power and signal routing was just too expensive for this project, so I had to find a simpler solution. Using large gauge wire seemed logical at first but stripping all of the insulation in the right place and soldering would be finicky. Copper tape came to the rescue; providing enough of a current capacity, in an extremely low profile. Not enough to carry all of the current on a single bus, but this many LEDs always works best with many small power buses since 5V doesn’t allow for much voltage loss over the length of the power run. The copper tape was laid out in 4 separate buses at the end of each row so that I could use very short jumpers to wire each strip to the bus. Soldering each jumper to took a bit of finesse, since each strip acted as a huge heat sink meaning heat had to be held to the joints for a long time, but not so long as to melt the adhesive or the acrylic beneath. I started by scuffing up the copper with some 100 grit sandpaper and then held my iron to the top of the wire (or the small bit of copper tape I used for the ground bus) and melting the solder into that from the top until it adhered to the bus.

Read more.

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4-channel I2C-safe Bi-directional Logic Level Converter: Because the Arduino (and Basic Stamp) are 5V devices, and most modern sensors, displays, flash cards and modes are 3.3V-only, many makers find that they need to perform level shifting/conversion to protect the 3.3V device from 5V.

We do have some other handy level shifters in the shop, from the DIP 74LVC245 to the fancy bi-directional TXB0108. However, neither of these are happy to work with I2C, which uses a funky pull-up system to transfer data back and forth. This level shifter board combines the ease-of-use of the bi-directional TXB0108 with an I2C-compatible FET design following NXP’s app note. Read more.


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