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I am designing a fairly simple board with an 5x8 LED matrix. I plan to drive it with an LED driver I haven't chosen yet, probably on a separate board connected by a ribbon cable.

My question is: what are the best practices for arranging tightly grouped LEDs on a board? If they're laid out like so:

x      x      x      x      x      x      x      x
x      x      x      x      x      x      x      x
x      x      x      x      x      x      x      x
x      x      x      x      x      x      x      x
x      x      x      x      x      x      x      x

you can see that the channels are going to be pretty tight. Anyhow, this thing just seems like there's got to be a standard solution already worked out ("arranged in a Hofstadter configuration", or something like that).

Also, is there a standard ribbon cable with 41 pins?

TIA.

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  • \$\begingroup\$ In case it wasn't clear...these LEDs are controlled independently and will not be multi/charlieplexed. \$\endgroup\$ Aug 4, 2010 at 16:11
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    \$\begingroup\$ Why don't you use multiplexing? \$\endgroup\$
    – starblue
    Aug 4, 2010 at 16:24
  • \$\begingroup\$ Well, I was trying to keep the complexity in the software, where it's easy for me. But it's looking more and more like it's easiest to go with a multi to reduce the connections. I'm just intimidated by it...looks like I need to get past it. \$\endgroup\$ Aug 5, 2010 at 3:15

4 Answers 4

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I'm using a Micrel MM5450... still having some small problems with it but bound to have it working on the next few days. It takes only 3 connections to your microcontroller (clock, data enable and data) using serial comm. and it can drive up to 34 LEDs.

http://www.youtube.com/watch?v=Shg0nCd6DRk

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There is not a standard ribbon cable with 41 cables. 50 is fairly common, and 40 is also common (IDE hard drives). The values (from wikipedia) are usually one of the following: 4, 6, 8, 9, 10, 14, 15, 16, 18, 20, 24, 25, 26, 34, 37, 40, 50, 60, 64 and 80.

I'd suggest going down to the 13 wires you'd need with a multiplexed array (or 7 you'd need if you were charlieplexing). You want to drive them with a pulsed wave for maximum brightness and efficiency. If you multiplexed them by rows and columns, you'd have a 20% duty cycle (5 rows) and an 8-output bus (easy even on an 8-bit micro)?

Also, you didn't mention what kind of PCB you were using. With a 2-layer board, assuming you can only fit 1 trace between your LEDs both vertically and horizontally, you can get all but the center 2 just by connecting the outer LEDs and slipping one trace past them to the next row from the top and bottom, and the 3 on each side of your middle row can come in from the sides. Work the last two in some other way, maybe on the ground plane side. (Note: I'm assuming you're using a ground plane.)

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  • \$\begingroup\$ Also remember that if you're multiplexing that you have to lower your current limit resistors. If the LEDs are capable of passing 20mA continuously and you're only driving them 1/5 (20%) of the time, you will want to drive them with at least 70-100mA of current in order to get decent light output. Just make sure (in software, or with a hardware watchdog) that you can't leave the multiplexed outputs at a static level, or that particular LED will be dissipating much more current than it's rated to. LEDs tend to go very bright, then dim, then sort of bright, then permanently black. :-) \$\endgroup\$
    – akohlsmith
    Aug 5, 2010 at 6:06
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What size LEDs? What type of case?

I did an 8x8 layout with 5mm LEDs. I equally spaced the LEDs in a 2.1" x 2.1" area. I would not space them much tighter than that.

You can see how the spacing looks if you take a look at the finished board and the drill template at http://wiblocks.luciani.org/PICO/PICO1TR-LED-L-index.html#drill

I also did a 10x10 layout of 3mm LEDs in the same 2.1" x 2.1" area. See http://wiblocks.luciani.org/PICO/PICO1TR-LED-S-index.html

Pictures of an example wooden case are at http://wiblocks.luciani.org/docs/app-notes/pico-led-drill.html

I do not know of a standard 41 pin ribbon cable. Most ribbon cables crimp on dual row headers. I have seen some ribbons for 37 and 50pin D-subs but I would stick to a header style. If you could live with 40pins you can use an old IDE cable.

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132 LED driver IC !!!!

I just heard (October 2011) on PICList (thanks Colin) about this IC . 132 LED driver from Austrian Micro. Digikey sells a number of their ICs but do not list this one as yet.

They say:

  • The AS1130 is a compact LED driver for 132 single LEDs. The devices can be programmed via an I²C compatible interface.

    The AS1130 offers a 12x11 LED-Matrix with 1/11 cycle rate. The required lines to drive all 132 LEDs are reduced to 12 by using the cross-plexing feature optimizing space on the PCB.

    The whole LEDMatrix driving 132 LEDs can be analog dimmed from 1 to 30mA in 256 steps (8 bit). Additionally each of the 132 LEDs can be dimmed individually with 8-bit allowing 256 steps of linear dimming.

    To reduce CPU usage up to 36 frames can be stored with individual time delays between frames to play small animations automatically.

    The AS1130 operates from 2.7V to 5.5V and features a very low shutdown and operational current. The device offers a programmable IRQ pin. Via a register it can be set on what event (CP request, Interface timeout, Error-detection, POR, End of Frame or End of Movie) the IRO is triggered. Also hardware scroll Function is implemented in the AS1130.

    The device is available in a ultrasmall 20-pin WL-CSP and an easy to solder 28-pin SSOP package.

Product page / General details here

Data sheet here

http://www.austriamicrosystems.com/eng/led-driver/AS1130

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