William "Chops" Westfield wrote:
> You will have trouble operating at 3V as "LED power" with
> UL200x style (darlington) drivers.  Darlingtons have about
> 1V voltage drop across each driver.  By the time you have
> one volt on the high side, and one volt on the low side, you
> have one volt left for your LEDs, and it's probably not enough.

There's something I misunderstand here. Now feel free to correct me, but 
here's my current understanding:

The ULN2003 lowside drivers work as follows:

* You have a darlington transistor
* The emitter goes straight to ground
* The collector goes straight to the output
* The base goes to a resistor that goes to the input

Now as I understand it:
* You put 5 V on the input
* Current flows into the base resistor, into the base, across the B-E 
junction dropping 700 mV in the process, and then straight to ground
* Because there's a base current flowing, this causes a collector 
current to flow (where Ic = Ib * Hfe)

If we connect the cathode of an LED to an output of the ULN2003, then 
current flows from the LED's cathode, into the collector, across the C-E 
function dropping maybe 100 mV in the process, and then straight to ground.

So therefore I don't see why you think I'll lose an entire volt in the 
lowside driver. I'm thinking 100 mV tops. But please enlighten me.

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