> Can anyone suggest a very simple NMOS transistor that's on 
> at 5 V and
> off at 0 V, and that can pass 30 mA? Or if you've got 
> another idea,
> please throw it out there :-D

There is, I think, a missing piece of crucial information.
Your requrement seems to indicate that there is a common 5V 
supply and that the switch&LED  and processor all operate 
from it. If so then the following answers make sense. If the 
LED has an eg 12V supply then you are going to need some 
form of level shifter as per recent discussions.

1.    The FDN338P is often a good choice for this sort of 
application.
Nominal 1.5V Vgsth switching on voltage but useful drive at 
2.5V gate voltage.Typically 0.25V "saturation" at 2A and 
2.5V drive.
Available Digikey at reasonable prices.

    http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&keywords=fdn338

2.    Using a BC327 as an emitter follower. This has 
opposite on/off polarity to solution 1.

Drive to base direct from PIC.
LED +resistor from emitter to V+ , collector to ground
OR
Resistor emitter to V+, LED from collector to ground.

The achieved current is "somewhat dynamic" [tm] and cannot 
be exactly designed [tm again] BUT is liable to be good 
enough.

In the first version the current is (V+-V_LED)/R.
As V_LED depends somewhat on I it will settle at a point on 
the operating curve and be dependant on LED parameters BUT 
will save you one resistor per circuit :-).

Second version is more arcane. A version of this is useful 
with an NPN low side driver where resistor is in 
emitter-ground and LED is in collector-V+ with V+ LED > 
V+_processor. This then approximates [tm] a constant current 
drive.



        Russell







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