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Topic: Sanity check - 8x8 RGB display with 8-bit PWM resolution (Read 5963 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.


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