> I'm using a ULN2003 in my current project to light LED's. 
> The ULN2003
> has 7 channels; I need 9 channels however. I could of 
> course get two
> ULN2003's, but I reckon it'd be more economical just to 
> get two NMOS
> transistors for the final two channels.
>
> Would an FDN337 be preferable for this?

Overkill. But would work well.


> Also, while I'm asking the question: Can the UDN2981 be 
> used exactly
> like the ULN2003? Or does it need resistors going into the 
> input? Are
> the inputs active-high or active-low? What transistor 
> would you use if
> you wanted an extra channel, FDN338 maybe?

You are certainly going to get adverse responses to that 
question, so let me jump in first.

That answer will give people the impression that you haven't 
tried to find out yourself, and that you expect other people 
to spend their time doing for you what you could do for 
yourself by looking at the data sheet.

NOW    it may be that the impressions given are incorrect. 
And they may not be. If you both do a small amount of 
research first and also mention even ever so briefly what 
you have done it lets people know that you are trying, and 
that you are not free-loading, and will encourage them to 
assist. it would also give people a better indication of 
your level of experience as, if you asked questions like 
those above after having had an honest attempt first off 
with the data sheet, then it tells people about the sort of 
approach that will be of most help.

Failing to not only do a moderate amount of 'due diligence' 
AND also to briefly state what you have done will have the 
likes of Olin and Jan wading in waving wet fish and 
muttering about phases of the moon, and worse.

So, all that said:

UDN2981

    http://www.allegromicro.com/en/Products/Part_Numbers/2981/2981.pdf

A useful device.
Low side in, high side out equivalent to ULN280x/ULN200x
Examination of data sheet says you need no Rin, can drive 
with up to 20 volts (25 one version) and that Vin may be 
able to be under a volt but that 2.4V is an approved value.


        Russell

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