Thanks,
Tom

On my furnace board, I used ON Semi's DTC144EET1G, mostly because I
wanted a SOT-523 (i.e. really small) package.  This is an NPN bipolar
with a built-in base resistor, since you need some sort of resistor
when driving bipolar transistors.  Then you have to do the math - how
much Ice current do you want, divide by worst-case Hfe (gain) to get
Ibe, determine voltage difference between worst case Voh of your
driver and highest Vbe, select base resistor.

For N-MOSFETs it's a little easier, as no base resistor is needed and
the turn-on voltage drop across the MOSFET is very little; you can
almost pick the resistor as if the resistor/led were connected right
to the voltage rails.  You just have to pick any N-MOSFET with a Vgs
turn-on voltage sufficiently below your minimum Voh, and a Ids current
capability high enough for your LED.

To recap:

1. Select a physical size, like TO-92 or SOT-23.

2. Choose between bipolar or MOSFET.

3. Current capability: For bipolar, select for Ice.  For MOSFET,
   select for Ids.  Choose at least 2x your expected LED current.

4. Pick the cheapest one that's still listed ;-)

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