I am planning a project in which I would use a Peggy 2 with RGB LEDs. Leaving the last row and last column unused, I am left with a 12x12 grid of 2x2 cells. One can put one (common-anode) RGB LED in each 2x2 cell. For greater brightness, I am considering putting two, three or four RGB LEDs in each 2x2 cell, and wiring them together in parallel (and reducing resistor values accordingly).

1. Any circuitry problems doing this?

I presume that, for each 2x2 cell, I need only use just three of the four anode pin holes and two of the four cathode pin holes (the latter pair being wired together). There are, of course, software implications.

2. Correct?

To facilitate construction, I am thinking of making 144 five-pin modules, one for each 2x2 cell. I would first solder the RGB LEDs on each module, then plug each module into the Peggy 2 and solder the five legs. Horizontal and vertical spacing (LED-centre to LED-centre) appears to be about 11.5mm.

3. Good idea? Suggestions for doing this? 144 copies of a custom PCB?

I plan to construct a hardwood frame, and may construct walls around each 2x2 cell with cardboard or wood veneer. If I do the latter I may cover the LEDs with a sheet of translucent material. If I leave it uncovered, I would use diffused-lens RGB LEDs.

Suppose I were to build the walls and cover the structure with a sheet of translucent material.

4. Any suggestions of what translucent material I might use? Something like this, perhaps: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170488974009&ssPageName=STRK:MEWAX:IT ?

5. Assuming I were to cover the LEDs, I'm undecided whether to use clear- or diffused-lens LEDs. Diffused-lens LEDs may provide a more pleasing dispersion of light within each 2x2 cell, but reduce brightness when compared to clear-lens LEDs. Opinions?

Thanks.

Cary

I am planning a project in which I would use a Peggy 2 with RGB LEDs. Leaving the last row and last column unused, I am left with a 12x12 grid of 2x2 cells. One can put one (common-anode) RGB in each 2x2 cell. For greater brightness, I am considering putting two, three or four RGBs in each 2x2 cell, and wiring them together in parallel (and reducing resistor values accordingly).

1. Any circuitry problems doing this?

Depends exactly what you mean. For "reducing resistor values," I imagine that you are referring to the current-setting resistors on the LED driver chips, not independent resistors on the LEDs.

Wiring up LEDs in parallel, without independent load resistors is usually a poor idea. The reason is that the forward voltage varies (a little or a lot) from LED chip to LED chip. Imagine that you take two red LEDs, each rated for 25 mA, and put them in parallel. Now, you drive the pair at 50 mA with a constant-current driver. But, the forward voltage for the two is slightly different. One reaches 25 mA at 1.8 V, the other at 2.0 V. So the actual current distribution when you drive the pair at 50 mA is that the current distribution is actually (say) 35 mA, the other at 15 mA. The first one goes out in a (proverbial) puff of smoke, leaving just one LED, now driven at 50 mA (!) and also goes out in a (second, still proverbial) puff of smoke. You can reduce the likelihood if this failure mode this by giving each LED a series resistor. (Also, driving LEDs in pulse mode with low duty cycle (like the Peggy 2 does) allows you to briefly exceed the "continuous current" limit-- the pulsed current to each LED can usually go to at least 100 mA, if you don't leave it on steady.)

There's not a whole lot of voltage "headroom" on the peggy if you're running at 4.5 V, but if you're running at 5 V, you could certainly put a 20 ohm resistor in series with each LED element wired in parallel.

The second issue to consider is that one *row* is switched on at a time. Having all three LEDs of an RGB element in one row would let you switch the three on with one common anode pin. Trying to split the element over two rows doesn't really make sense, because you lose the ability to control the elements in two rows, same column, independently.

I presume that, for each 2x2 cell, I need only use just three of the four anode pin holes and two of the four cathode pin holes (the latter pair being wired together). There are, of course, software implications.

2. Correct?

I think you need just one of the anode pins (the common), and three cathode pins, if it's a common-anode RGB device. Again, splitting one element between rows isn't a good idea. I'm not sure how you get to this five-pin situation.

To facilitate construction, I am thinking of making 144 five-pin modules, one for each 2x2 cell. I would first solder the RGB LEDs on each module, then plug each module into the Peggy 2 and solder the five legs. Horizontal and vertical spacing (LED-centre to LED-centre) appears to be about 11.5mm.

3. Good idea? Suggestions for doing this? 144 copies of a custom PCB?

Yes, you could lay it out as a custom PCB, you may even want to consider making one new Peggy-derived board with all of the "modules" on board.

I plan to construct a hardwood frame, and may construct walls around each 2x2 cell with cardboard or wood veneer. If I do the latter I may cover the LEDs with a sheet of opaque material. If I leave it uncovered, I would use opaque RGB LEDs.

Suppose I were to build the walls and cover the structure with a sheet opaque material.

4. Any suggestions of what opaque material I might use? Something like this, perhaps[...] ?

Yes, polyethylene will work, as will nylon, or diffused-white acrylic. PE is a bit more opaque than you'd like, probably.

5. Assuming I were to cover the LEDs, I'm undecided whether to use clear or opaque LEDs. Opaque LEDs may provide a more pleasing dispersion of light within each 2x2 cell, but reduce brightness when compared to clear LEDs. Opinions?

If you plan an external diffuser for the LEDs, it's usually best to use clear. If you don't, then use diffused. Diffused LEDs are best when you look *directly* at the LEDs. For all other situations, use clear.

Many thanks, Windell, for the very complete and super-fast reply. I now understand the problems and should be able to work something out. After doing so, I'll post a follow-up to this thread.

Just one point of clarification. In reply to my question #1, you said,

"The second issue to consider is that one *row* is switched on at a time. Having all three LEDs of an RGB element in one row would let you switch the three on with one common anode pin. Trying to split the element over two rows doesn't really make sense, because you lose the ability to control the elements in two rows, same column, independently."

I did have in mind giving up independent control of the two rows in each 2x2 cell. If, for example, if I had four RGB LEDs per cell, I would intend that all four red LEDs would be on or off together, at the same brightness. Is that consistent with the point you were making?

Cary