Electronics help, anyone?

[EightBySix]

Hi Guys,

Wonder if anyone can help me with my electronics sub-project for my mame coffee table. I've self taught myself as far as I can, but I've got stuck.

Dunno where its best to post this. Didnt want to clutter up my project thread and thought if I keep it seperate so it might go in an electronics forum, should one get created *cough*.

I want to make an LED matrix display, like the ones you see scrolling information in shop windows etc.

So far I've:

• Got my self familiar with programming PIC micros
• Bought a bunch of 7x5 LED matrices
• Learned about multiplexing the LEDs and using shift registers to reduce the number of pins needed.

My next step is to control 1 LED matrix via the PIC, and then extend it to the rest of them, by cascading the shift registers.

I understand that I'll need to get some transistor arrays, as the PIC can't provide enough current to light a whole row of LEDs (for all of the LEDs)
Also, that I'll need to get shift registers that can sink enough current  (I've seen some LED driver shift registers, that seem to have extra features, such as in built current limiting resistors, dimming features, etc)

What I'm struggling with, and have failed to find much on Google about (probably because I dont know what to search for) is how to get enough power to light a whole row (5 LEDs 8 10 matrices). Obviously, my battery wont cut it - so what do I do - do I get a power supply with a high current and divide it between the marices? How would I do that?

I guess the knowledge I need now is less digital electronics, which I can get my head around, and more into equations about current and voltage etc, which I've had no practice with.

Any advice? Even if it's pointing me at some suitable reading :-)

[MonMotha]

TI has a nice series of "dumb" logic devices with power outputs.  Check out the TPIC6C595 (and similar A and B versions) for this.  That's what IT used on their Golden Tee signs.  You can easily drive them off a SPI controller to create a really long chain for a big matrix like 40x7 or even bigger.

You'll probably want to high-side switch the rows (using PNP transistors or P-channel MOSFETs) and low side switch each column using the above-referenced devices.

As for power, remember the instantaneous power requirements are only the LEDs that are on at once, i.e one full row at worst.  So if you've got rows with 40 columns, and each LED needs ~30mA, you need 40*30 = 1200mA = 1.2A.  Each column needs its own bias resistor or current control loop (the latter is far easier and sufficient in this case).  Probably easiest to just use a 5V supply, so you need 5V @ ~1.5A (to give you some room for your driving electronics).  This is actually a fairly modest 7-8W supply.  A PC supply will easily handle this, but I'd actually shy away from doing that as you're likely to melt things if you mess up.  I'd just find an old wall wart.  If you want to use batteries, you can use 3x D cells for ~4.5V, and they'll easily source the needed 1.2-1.5A.

[EightBySix]

Thanks MonMotha - thats just the sort of help I need.  Got me some more googling to do now

[ids]

   I wanted to reply, but my knowledge is at that dangerous level...not completely absent, but not sufficient to stop people from lighting themselves on fire.  I want to help but....   Kudos MonMotha

[MonMotha]

Just to correct myself from earlier, I meant to say that the resistor is easier and sufficient for this purpose (which would be the "former", not the "latter" in the above context).  At these power levels and to simply bias an LED or two, there's little reason to mess with current feedback loops or switch mode supplies.

[BobA]

See attached thread for DIY digital signage.

Your whole array of leds should not be powered at once since the scrolling of letters and changing displays require the LED arrrays to be multiplexed.   Do not calculate power by powering all leds with DC and adding them up.   Your method of multiplexing has to be determined first to see what percentage of the time an column of LEDs will be illuminated.

Make Mag

[EightBySix]

BobA, I don't know whether to kiss or punch you! That is exactly what I need, only I have spent so much time learning how to make my own i'm kind of gutted that so much has been done already, and way cheaper tha I could have made it for, especially once I cost out making pcb boards etc.

Looking over the spec of the display boards themselves, it still leaves plenty of scope for tweaking and programming though, and will leave me more time to actually make the kit that's going to house it. I'll update my project thread when they arrive.

[BobA]

Have fun.  Looking forward to seeing what you come up with.   

No punching or kissing please.