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I have a client who has several large bronze signboards, each with 100-150 little light bulbs on it, totaling approximately 400 bulbs. The bulbs are turned on and off on a schedule, currently by hand (partially unscrewing the bulbs that are not needed). Most of the time, approximately a dozen bulbs will be illuminated at a time, though there are a few days every year that all 400 need to be on. The current bulbs are 120 volt AC, with the neutral connection made through the bronze frame of the board.

I want to replace all of the bulbs with LEDs to reduce power usage (and shock risk from touching the bronze board and a ground) and to schedule the turning on and off of the lights. I saw this question about controlling multiple LEDs, but it doesn't sound like that solution will scale to the hundreds of bulbs that I need, both because of the number of connections and chips needed and because of the current required (even though I usually only need a dozen or so lamps on at once; the few days that I need all of them at once will require a lot of current).

Does anyone have any ideas for how to do this? If I have to, I can control each of the boards (with 100-150 lamps) from a separate device, but I would prefer to do them all from one.

UPDATE: I should say that, although I have looked at several LED voltage/current options, I'm not committed to anything yet (and I haven't bought anything yet), so I can choose what I will get based on whatever fits best in this situation. The only requirement I have right now is that the LEDs must be orange, to match the color of the old bulbs (though they don't need to be exactly the same).

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Moshe Katz
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  • What type (voltage/current) of LEDs are you planning to use? – John La Rooy May 19 '13 at 20:37
  • @gnibbler I suppose I should have put that as part of the question. I have several options, but I haven't committed to anything yet, so I have no problem switching to whatever will work best. – Moshe Katz May 19 '13 at 20:39
  • Ok, do you know roughly how many watts they need to be? The more specifics you can give, the more useful the answers can be :) – John La Rooy May 19 '13 at 20:42
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    So just regular orange 5mm LEDs at 20mA? – John La Rooy May 19 '13 at 21:42
  • @gnibbler Yes, that's the general idea. – Moshe Katz May 22 '13 at 17:15
  • I'm probably going to end up using a WS2801-based setup because there is a lot of documentation out there for it and it is specifically designed for this purpose. – Moshe Katz Jun 26 '13 at 04:57
  • Wow, you'll need one chip per 3 LEDS. I'm not sure why you'd use those if you just want on/off. Are you planning on using the PWM for something in the future? – John La Rooy Jun 26 '13 at 05:18
  • I didn't say it was optimal. I'm sure it's not. If I end up doing it that way, it will be because I was able to find premade strings (RGB ones, but I don't need that) that use it which I can use as an example to build mine. I feel much more comfortable doing it that way. I also have not made a full decision on that though. I'm still looking for a good example somewhere of using the 4094 that you mentioned. I haven't found one that I'm comfortable following yet. – Moshe Katz Jun 26 '13 at 05:33
  • Another option is TPIC6B595, that's one chip per 8 LEDs. You should experiment to find out which LEDs you need and at what current. If 6mA is bright enough, you can even just use an 74HC595 (much cheaper). If you can't get them to work, you should ask a new question specifically about that. You can try sampling some from ti.com, so it's low risk. If you can daisychain 2 chips the principle is exactly the same for 50 – John La Rooy Jun 26 '13 at 05:48

2 Answers2

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Depending on the LEDs you are using, It may be possible to use a multiplexing scheme to reduce the number of drivers you need.

It's probably simpler to use a bunch of 4094 shift registers daisy chained. You can bitbang the clock/data or use the SPI hardware to drive them

Each 4094 can control 8 LEDs. You'll most likely need a small driver (eg transistor) per LED if you do it this way.

If there is a regular layout, you will save a lot of work (and opportunities for mistakes) by making a small PCB that can be tiled together.

John La Rooy
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There is some documentation available on Swarm Light, which a design installation featuring a large amount of individually controlled LEDs. They used 1 microcontroller for each set of 3 LEDs and six more to communicate with each of the 300 "sticks" of 10 controllers via serial interfaces. Perhaps your 400 LEDs are many enough to require such an architecture.

Bengt
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