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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).

  • 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
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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.

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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.

  • That looks way more complicated than what I need. All I need to do is on/off, every few days. – Moshe Katz May 22 '13 at 17:13
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    Swarm Light updates the light values at 120 Hz. So yes, that might be overkill. – Bengt May 22 '13 at 18:09

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