LED Matrix Backpack

I had purchased some huge (2″) 5×7 matrix a while ago, as part of my led sensor research. They’ve basically been banging around the lab since, getting pins bent and such. So this past weekend, I decided to put them to some other use. Their pin layout is sort of weird, it doesn’t match up with a breadboard at all (one of the reasons they never made it into the led research). So, I decided to make up some back-packs for them, or is it a carrier board? Anyway, the board features one 5×7 matrix, one pic 16f737, a few transistors and some data connectors. The board provides two means of serial communication; asynchronous rs232 at 19200 bps, or synchronous i2c at 100kbps. A second connector provides power and ICSP pins.

5x7 led matrix backpack

This project has no practical application as of yet. The main reason I made it was to improve on my double-sided pcb fab techniques. This time I found using point to point traces instead of a large “pour” made things work a lot smoother. I used the ‘sandwich’ method with press ‘n’ peel blue. Roughly 1.5 min per side.

5x7 serial led matrix backpack

One thing I had to keep in mind while doing this layout was accessibility to solder both sides, since I can’t through plate my own vias. So things had to be laid down in specific order. I soldered the vias first, using some cheap resistors with very fine leads as my conductors. The method involved sticking the resistor into the via, with just a bit poking out the other side, then bending the resistor 90 degrees and holding it to the board. Then a quick dab of solder onto each joint set them in place. Next, straighten out the resistor leads, and trim them off. Another quick few dabs with the soldering iron and each one was fully connected. Next came the smt parts. The switches were rather easy, but those blasted little 0603 capacitors always give me grief. I tin both pads, then apply a bit more flux as “glue”, then try to reheat one of the pads, to reflow the solder onto the part. It works great with 0805 and larger parts, but the little 0603 usually gets sucked onto the tip of the iron by the surface tension of the solder.

One hard choice I had to make was whether to solder the chip straight into the board, or use a socket. I opted for a socket, which meant a harder time soldering the “top” layer. Luckily, I was able to dig up some 14 pin machine pin sockets, and thanks to the machine pin itself, they stand proud of the board a little, just enough to sneak in with the soldering iron.

For whatever reason, I decided to solder the led display next, leaving the connectors for last. I partly wanted to see which side of the board the connectors would look better on… I think next revision, they’re going on the bottom. During assembly of the connectors, I nicked the display a couple times with the iron, oh well!

Right now, the display is flashing my initials. Oh, here is the schematic, nothing exciting really!

quick video:

6 Replies to “LED Matrix Backpack”

  1. I see your code is in assembler, which is beyond my understanding. Essentially you need to talk to a shift register like it is an SPI device. There is a ton of shift register example code (in ASM) on google.

    It would make life a lot easier if you simply upgraded to a bigger processor. I’m using a 28pin pic so it has enough direct IO lines to manage the matrix without any support chips.

    You could also look at using a 3 to 8 decoder, like the 74HC137 or 74HC138, now it becomes as easy as writing a 3 bit value to three pins in order to select one of eight outputs.

  2. can i ask, what are materials needed for the led designs with scrolling? can i ask also for the circuits? i am making my project for project design.. ty

  3. Gaddi,

    The circuit for the scrolling sign is the same as seen here – it’s just better software in the mcu which does the scrolling. the materials are simple, a mcu, an led matrix and some transistors.

  4. Gaddi,

    I used P channel mosfet transistors in a super sot-3 package (same as sot-23). You could also use standard pnp bipolar transistors, remember to include a base resistor. The exact transistor is not important, the only thing to watch for is that it can handle an entire column of current, for example 7 * 25ma.

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