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Brad Fitzpatrick

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More xmas light thoughts [Nov. 26th, 2003|12:34 pm]
Brad Fitzpatrick
Yeah, using a parallel port to drive the xmas light grid would be much easier. Thanks for all the recommendations. And the parapin library makes it easy.

So, I can get a 74LS74 TTL component which has two D-flip-flops for like $0.60.... need then like 45 of those. That doesn't cost much. Will also need a transistor for each flip-flop too... ideally I could find a TTL component with both a flip-flop and transitor in it.

Guess I need to run constant power down both the bottom and right sides to power the flip-flops, and then a separate power down the bottom which I turn on and off for the single pixel that's lit at a time. I wish I had a good tool to draw a schematic with.

Haven't thought about packaging at all.... it's all going to get wet. If I have ~25 of these TTL components every 6 inches down the side, maybe I can drown them in epoxy blobs? I'd prefer to do that than run a ton of wires from the base. This all needs to take apart well and connect in pieces.

My calculations about bandwidth before were totally wrong. I'm basically scanning a single dot over the 54*40 grid and conditionally lighting it at every location.... so that's 54*40 = 2160 locations... each location will take a write from the parallel port to setup the signal (with clock low), then another to bring clock high, then another two to optionally light/unlight the pixel.... so that's, what? 8640 signal changes per screen? And the parallel port can do like 11 Mbps (more than USB 1.0, I recall?) with 12 in/out pins? So 1 Mbps for a single pin? Um, should be plenty fast.

Man, I've been re-reading my college electronics book. Things are slowly coming back, but I really don't know what I'm doing. I suppose I should go pick up some parts and start tinkering with a small-scale version.

[User Picture]From: brad
2003-11-29 02:16 am (UTC)
I think you'll find the lights have plenty of hysteresis so your scan rate won't need to be fast.

Yeah, that's what it's looking like.

I'll look into FETs, since I have no clue what they are.
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[User Picture]From: ch
2003-11-29 12:15 pm (UTC)
fet's are used to switch dc.

to switch ac you'll need to use an scr (half-wave) or a triac (full-wave aka bilateral).

there are logic-level triac drivers available that make triggering triacs from logic trival.

there are even some really nice opto-isolated parts, e.g.:


isolation is a good idea to keep from toasting your computer if something bad happens.

p.s. if you're mail ordering parts, digikey is one of the best.

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