How to build a massive LED array

June 26, 2006 11:02 AM

Continuation from my last question: how do I make a giant red "M" out of little LEDs?

After reading all the answers from my previous question, I became enamored with the idea of building the big red "M" out of LEDs. The only problem is, I know about as much about LEDs as I do needlework, which is to say, nothing at all.

Can someone point me in the direction of a tutorial that will help an absolute noob like me learn enough about LEDs to build the "M"? This is going to be a massive array of LEDs, so the "build your own LED flashlight" articles aren't really helping me.

posted by bjork24 to technology (20 answers total)

1. Figure out just how big you want it to be.
2. Buy a bunch of Busboard, like this stuff:here
3. buy a bunch of red LEDs. Read the power specs on them, hook them uop in parallel on your bus board, and provide a power source that's withing spec for them. Usually, this involves using a resistor to drop the power into spec. You can probably use a switching power supply to get low voltage DC and then wire up the resistors on the busboard.
4. After you figure out how to make them light up, spend hour and hours popping them into the bus board and soldering them into place. I would suggest avoiding the LEDs for a project of this scale. If you want to make it look like LEDs from a distance, just put a piece of peg board in front of a normal florescent lit "M". Then you'll get the pixerlization of the LEDs without the price and headache.

posted by cosmicbandito at 11:22 AM on June 26, 2006

For a really large set of LEDs like this you probably want to make a bunch of serial strings and then wire these in parallel. You can then run them at 24 VDC or something similar. There are lots of semiconductor companies that make LED drivers that make this easier.

Sadly, I don't know of a resource that can walk you through this step by step. One option would be to find a local electronics geek that could help you. Another option would be to find a partially preassembled solution, for instance something like this, but in red (for some reason, sparkfun doesn't have red LED bars, but you could always shoot them an email).
posted by pombe at 12:29 PM on June 26, 2006

Here's a red led bar option that might be exactly what you want - just provide it with 12V DC, and you're done. Assemble as many as you want in parallel, just make sure you have enough current to drive them all.

Post photos when you get it assembled!
posted by pombe at 12:31 PM on June 26, 2006

Gargh! This is turning out to be more trouble than it's worth... back to the flourescent drawing board.
posted by bjork24 at 12:37 PM on June 26, 2006

Non-local electronics geek at your service if you decide you want to pursue it...

email in profile
posted by FauxScot at 1:09 PM on June 26, 2006

It won't be much trouble electrically..

Red LEDs take about ~2v to turn on. Brightness varies with current, but with enough current the LED will eventually burn. Try designing around 20mA, and see how you like it.

So, if you want 20 LEDs powered together, you have two choices. You can wire them in parallel, which will require a ~2v supply at 20 x 20mA = 400mA, or you can wire them in series, which will require a ~40v supply at 20mA.

To limited the current, and to keep the brightness even, you should have a resistor in each parallel branch of the circuit. For example, with 20 LEDs in parallel you will need 20 resistors, with 20 LEDs in series you will need one resistor.

For safety, price, and availability you will want the voltage to be less than ~40v, and the current to be as low as possible given the voltage constraint.

All this means, figure out how many LEDs you are going to want. Find a power supply that can provide more than (# of LEDs) x 0.08W of power. Based on the voltage of that supply figure out how many LEDs you can chain together in series. Build your chains (including a current limiting resistor chosen by measurement and testing to protect the LEDs from heat death while providing an appropriate level of brightness). Attach your series chains in parallel to the supply, turn on, cool look the red M!

You will need a multimeter and a soldering iron.
posted by Chuckles at 1:14 PM on June 26, 2006

What about something like a giant Lite Brite M? Granted, I know absolutely nothing about the technical aspects of making such a thing happen, but it doesn't seem like it would be hard. Make the aluminum M, light it with fluorescent bulbs, put some kind of black material (cardboard?) over top and instead of LEDs, use a whole bunch of red Lite Brite pegs (I know you can get a ton of them on ebay). Then cover over the whole thing with clear plexiglass to protect it.

Just a thought. Disregard the above if it's totally implausible.
posted by amro at 1:16 PM on June 26, 2006

I'll make this simple, and I'll revert to my original idea:

Part I:
1. Procure pegboard(s) from home improvement store.
2. create shape of M in pegboard.
3. Draw/mark/organize where you want to put your LEDs.
4. Count marks, order requisite number of LEDs from internet.

Part II (this is where it gets more difficult, but not too difficult):
1. Divide M into 3 or four parts:
1a. If the middle of the M (the two parts that come to a V in the middle) is narrow, you'll divide into three parts (left leg, right leg, middle leg)
1b. If the middle of the M (the two parts that come to a V in the middle) is wide, you'll divide into four parts (left leg, right leg, middle-left leg, middle-right leg)
1c. Ideally, you want to create "runs" of identical lengths. So if the LEDs are 4 wide on each leg, and the middle is narrowish, you'll have 12 runs of, say, 20 LEDs.

2. Place LEDs in holes, alternating in each run the short/long wire from LED to LED (in other words, all the short wires go on the left, for instance).

3. Do what Chuckles said. In my example you'll have 12 runs of LEDs in series.

This is not difficult.
posted by Kwantsar at 1:25 PM on June 26, 2006

12 runs of LEDs in series

(for which you'll need 12 resistors, total)
posted by Kwantsar at 1:27 PM on June 26, 2006

Procure pegboard(s) from home improvement store.

I like it, sort of.. The various types of fiberglass protoboard made for electronics prototyping will be somewhat expensive for this job (such a huge area).

Keeping all of the LEDs perfectly straight and evenly spaced will be important. Obviously messing up the spacing will look shoddy. However. brightness varies somewhat with viewing angle, so too much misalignment will be bad too.. Electronics protoboard is good for this aspect of the design, but pegboard might be able to work well too.

Pegboard is often built with holes at 1" spacing. 0.5" spacing would look much better.
posted by Chuckles at 1:45 PM on June 26, 2006

This looks like 17" x 4.5" proto board for $5 a piece, which is a lot cheaper than I expected.

Assembly will be interesting.. After all that work, you will want to be able to swap an LED. They are very reliable, but you'll have hundreds, so I wouldn't rule out the occasional burned out bulb. However, much more likely, the wiring will be failure prone unless you make it very secure.

Here's a suggestion using protoboard.. Lightly glue the LEDs to the front of the protoboard and let the glue dry. To connect the circuit, bend the appropriate LED leads together and twist them around one another a few times in a pigtail, then solder.

This will get the alignment perfect. If you need to change an LED it will be easy enough to snip the leads, and then break it free with a pair of pliers. With the right glue it should be very weather proof. On the other hand, too strong a glue will fracture the protoboard when you remove an LED, so that is worth testing.
posted by Chuckles at 2:03 PM on June 26, 2006

The boards linked by Chuckles are far cheaper than I would have dreamt, and would work quite well.

One thing I failed to make clear was that the LEDs almost always have different-sized leads. Since they are diodes, current goes through them in only one direction. When you assemble them in series (for the "runs") the short meets the long of the next resistor, and so on.

Frankly, I'm not sure that you even need to solder the LEDs together in the middle of the runs. Part of me thinks that if you leave the leads long enough that they'll twist together and stay that way.
posted by Kwantsar at 2:20 PM on June 26, 2006

One thing to keep in mind is that LED brightness and color is proportional to the current through the LED, not the voltage. LEDs vary quite a bit in their forward voltage, so using fixed resistors to control the current means that an array of LEDs will have varying brightness and color. This is not noticeable when you have just a few LEDs, but an array will look blotchy.

If this is just a hobby application you may not care. But commercial arrays like you see in sports stadiums use microprocessor controlled LED drivers that allow you to program the dot brightness correction for each LED (or series string of LEDs). For example this device from Texas Instruments.

In addition, the resistors are an inefficient way to control current. If you use a 5-volt supply across a single LED, more than half the power is just heating the resistor. At one-twentieth watt per LED, this can add up fast for a large array. For example a 100 x 100 array is 500 watts. More efficient drivers use semiconductor controlled current sources.
posted by JackFlash at 5:21 PM on June 26, 2006

Did I mention I'm a total noob?
posted by bjork24 at 6:30 PM on June 26, 2006

That Texas Instrument article is pretty interesting.

One thing to keep in mind is that LED brightness and color is proportional to the current through the LED, not the voltage. LEDs vary quite a bit in their forward voltage, so using fixed resistors to control the current means that an array of LEDs will have varying brightness and color.

Hmm...

As long as the problem is due to forward voltage variation, long strings of LEDs should be relatively okay - because the variation will average across many LEDs. However, the TI article says that the brightness varies across LEDs even when the current is held constant. Perhaps I'm missing something, but that suggests to me that the variation is due to other factors (or at least, other factors as well). Perhaps the mechanical alignment of the reflector, or the lens, or whatever..

Here is an application note that suggests that controlling for forward voltage variation is worthwhile: LDO Linear Regulator Improves White LED Brightness Matching.

Here is a more specific application note on Electrical properties of GaN LEDs & Parallel Connection. Again, it only concerns itself with variation in forward voltage.

It would be very straightforward to drive each string of LEDs with a current source, instead of just dropping a resistor in series with them. This would completely eliminate the forward voltage variation issue. It would also be very easy to change the resistors to current sources at a later date, if you decide the matching isn't good enough after it is built.

If the variation due to factors other than forward voltage is a problem, the fix will be more complicated than it is worth..

Don't let all these details discourage you bjork24. There are other lighting alternatives, but I think LEDs are the right solution for your sign.
posted by Chuckles at 6:51 PM on June 26, 2006

I wouldn't call this a total noobs guide, but it does seem to be basic yet thorough: Notes on LEDs.
posted by Chuckles at 6:54 PM on June 26, 2006

Maybe a bit too thorough.. I'd skip down to the heading "Using LEDs", which is about half way down.
posted by Chuckles at 6:57 PM on June 26, 2006

Hmm, looks like that Notes on LEDs page is a complete rip-off of this one: Notes on LEDs.
posted by Chuckles at 7:00 PM on June 26, 2006

For the technical-minded of you, a nice solution to the varying brightness issue is to put your LEDs in series strings, regulate the current in one with a resistor, and use current mirrors to regulate the rest of the strings (as shown here).

My apologies for intimidating you, bjork24, but I'm really into LED geekery.
posted by pombe at 9:17 PM on June 26, 2006

There's a great project on Make's website, which might be too small for the letter that you're considering, but it's worth looking at.

Also, maybe this will help you?
posted by hooray at 9:33 PM on June 26, 2006