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[Reply #15 on:]

Specifically which 5V? The VIN pin is a copy of the "raw" supply, ie what you put into the barrelconnector (minus a diode). You should use 7V - 12V there, but my experience is that 5V works (even though that means the 328P chip gets lower voltage, but it seems to run fine). So if you meant "split off" the WallWart/External supply, you can do so at this pin.

The 5V pin is after the regulator or the USB supply. There is roughly 500mA available there (less is preferable). We still do not know how much mA those LEDs actual do require...?

[Reply #16 on:]

Gotcha, that's what I was looking for, thanks. I haven't gotten the LEDs yet, but the diodes that Chagrin linked pull 30 mA, so I'll probably be dealing with something similar. I looked at the TLC5916 chip, and can't find any mention of how/where to use a resistor for output current, but I'd prefer to do it that way for simplicity. Any insights on how that works?

[Reply #17 on:]

We are talking about http://www.ti.com/lit/ds/symlink/tlc5917.pdf ?

It says on the front page: The constant sink current for all channels is set through a single external resistor. So I flip forward to a pinout spec - page 4 - where it says R-EXT External Resistor - Connect an external resistor to ground to set the current for all outputs. The diagram shows that as pin15. Zipping forward looking for a curve or formula for R-ext (it is a PDF and there is a search function in the Reader) - where I find on page 15 a nice chart and a summary: the default current is approximately 52 mA at 360 Ω and 26 mA at 720 Ω. At least unless you program the VG (Havn't bothered to find out how)

SO if you put a 1K resistor it is down to 20mA.

Yes, reading spec sheets is difficult, one needs to try it, often, re-read it several times.