Building an LED Matrix
I've hacked quite a few matrix displays over the past year, and now we've run out (not really, we have some from Limehouse, bur for the purposes of this intro, let's pretend we've run out). What can you do if you've run out of displays? Well, you have to start making your own. A while back I found a drawer in the Electronics Area containing a large number of 2x2cm 8x8 LED modules and immediately though "Hmm, I could make a display with these"
Then I forgot, until a few weeks back where I took one home to figure out the pins. They are pretty much just 788AS modules, with a weird model number which doesn't find much on the web. Once the PCBs and solder stencil arrived, I realised I'd forgotten to order the shift registers. Once those arrived, I started building (this was last night, Wednesday).
I used the four "spare" (they will be used!) boards out to hold the main board securely, then added aligned and taped down the stencil, then covered it in solder paste. That sounds messier than it was, it was relatively controlled.
And then, into the oven! This was a new purchase for the hackspace. It's the T-962A+, which is newer than the A model, which was newer than the original T-962A. It's controller is completely different to the previous versions, so we can't install custom firmware (yet?) - but it's actually proven itself to do a pretty decent job, especially considering the reputation these ovens have earned over the years (not great!). The issue is though, there's nothing really in between one of these and a very fancy £10k machine. Anyway, that's not what this post is about…
Placing 128 resistors for the two sets of columns was not as bad as I thought it would be. I listened to most of the French 79 "Joshua" album, and found it pretty relaxing.
Almost all the joints came out perfectly! The ones which did not were my fault. I smushed a shift register, and the WizNet chip. The ARM chip came out perfect, no touchup required. I soldered in a few of the 8x8 modules to do some testing. No point soldering them all in until I was sure it would work.
The first issue I had was that, while I could turn rows on or off, I
was not able to address columns. Whatever bit I pushed first got flooded
across the whole row. Well, I eventually figured out that I'd
accidentally mixed up my LATCH and CLOCK lines
in the schematics. Oops! But no harm done, they are just pins on a
microcontroller.
Even after figuring that out, it looked weirdly blurry. After an hour or so of probing about, I realised my error.
Do you see that? The ground… is not… connected! Noooo! Anyway, after a bodge wire, it worked! I soldered in the rest of the modules this morning, before work.
I realised I can clean that up a bit more, and scraped off the solder mask next to the pin. A quick blob of solder and you'd barely know!
It's been a cool project. I'm not sure where I'll put them. I'll keep one myself and put the rest around the hackspace in random places. This evening I tried streaming using ffmpeg and got the pixel PWM working pretty nicely.
More display posts coming in the near future I'm sure. Limehouse Labs sent us four 64x16 RGB panels, and I've been having some fun with those (also streaming Contact to them…)
Related posts:
Wanting to leave a comment?
Comments and feedback are welcome by email (aaron@nospam-aaronsplace.co.uk).