Brian Scearce wrote:
>
> I hate to make my first post to this list a classic rookie question,
> but I really want to get working on my project over the Christmas
> break.
>
> I have eight banks of eight LEDs arranged in a common cathode array.
> My original plan was to use a 74HC138 as a bank select, and wire PORTB
> to the anodes through current-limiting resistors.
>
> The 74HC138 can only sink ~25mA, so I was planning on presenting only
> one bit at a time on PORTB.  I did some more calculations, and I find
> that this approach will be too slow.
>
> So, I want to be able to sink more current (20mA/LED * 8LED = 160mA).
> This is where I start to get out of my depth.  I'm thinking eight PNP
> transistors (2907?) with the bases attached to the 74HC138 outputs, the
> emitters attached to the banks' cathodes, and the collectors grounded.
> But I don't know if I need current limiters between the transistors and
> the 138.  I don't know if I've chosen the right transistor for the
> job.  I don't know if the information presented in, say, the Digi-Key
> catalog is enough for me to pick the right transistor, or if I need a
> data book.  I don't know which data book covers these parts.
>
> I'm not describing my problem in order to have someone present me with
> a solution, just to describe my level of expertise.  The sci.electronics
> FAQ recommends _The Art Of Electronics_ (2nd ed), by Horowitz and
> Hill.  Is this a good book for someone at my level?
>
> Thanks,
> Brian

The _Art Of Electronics_ is an astoundingly great book.  If you put this
on your shelf, along with Don Lancaster's _CMOS Cookbook_ and _TTL
Cookbook_, and you read'em, you will be a better EE than most of the
degreed EEs out there.

Al Cohen
Boston Medical Technologies
(BSEE Cornell '85)