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Topic: Sanity check - 8x8 RGB display with 8-bit PWM resolution (Read 2 times) previous topic - next topic


You could use the ULN2803s to turn off the current source for the LEDs tho, as in this crude example.
Bit more power hungry this way, the LEDs are either seeing the current or the ULN2803 output stage is seeing it.
Seems simpler at times than trying to set up switchable current sources ...

Hey CrossRoads,

Looking through your posts in this forum since you made this comment, I was starting to think that you had shares in TI seeing as how you're such a regular proponent of using them in circuits. I've just been pricing up some of my different options to potential designs (they've all ended up fairly standard matrix display circuits for rev1, maybe some tweeks in rev n+1) and come to the conclusion that the ULN2803 is pretty awesome.

The two options I was looking at in the end

Circuit 1

Circuit 2

  • 3 x 74HC595 for column select

  • 3 x ULN2803 for sinking the column select

  • 1 x 74HC138 for demuxing the row select

  • 1 x ULN2803 for sinking the unselected rows

The problem is that finding any low power MOSFET (let alone a p-channel one) in a sensible package at a price that compares to 9.4p per channel on a 2803 is really difficult. Farnell have SMT ones at 9p, but hand soldering eight of them is not my idea of fun. I've found them in TO-92 packages, but with prohibitive shipping charges and not from my usual suppliers.

Damnit, I've spent all this time typing out a post and I've just run the final numbers and (assuming 25p for a TO-92 p-MOSFET) there's only 25p difference in price :( (£4.87 vs £4.61 respectively - mostly because of the column drivers being one chip rather than two).

So, even though it's a bit more hastle, I'm going to go with circuit 1. I'll probably steal a magnifying florescent light thingy from someone and hand solder a bank of tiny units for prototyping. This way, if I ever get a run of boards made they'll have the right chips on.

If only life was straightforward, then we wouldn't need internet forums to vent on.



polhemic if you could get the board your talking about up to 350/400 mA for 1 watt rgb led's i would be interested.

Hmm,  it almost sounds like you'd need some kind of current amplification device. I'll call the 50's and see if they're still carrying stock :)

Shouldn't be too much hastle to add on, but there might be better driver solution chips out there - I know that NXP have made more recent changes to their portfolio that 5940, so a quick bit of research might be in order



polhemic if you could get the board your talking about up to 350/400 mA for 1 watt rgb led's i would be interested.


... and Mike, that video is awesome. You deserve some kinda Make BAFTA for it.

I was watching thinking, "meh, no PWM here it's all just red, green and blue booooring" and then you took off with the light show. Top work.



@EVP: Looking at it going with the TLC5940 or the NXP equivalent is the smart way to go, it's just a shame that it bumps up the BOM by £33 just in silicon.

I'm kinda tempted to draw up some boards, send them off to PCB train and start selling daisy-chainable 16-channel RGB 120mA per line driver boards if I thought anyone would buy them. I think the I2C addressing lets you get a fair few in on one bus without any hastle. Like the Octobar LED controllers (http://arduino.cc/forum/index.php/topic,62510.0.html) but better because I made them :)

For the moment I'm going to persevere with a 74138 / p-chan MOSFET as the row driver into three darlington arrays as the sink, until I blow out the darlingtons and have to replace them with TLC5916 current sinks.

Much more fun to be had.


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