Yes, the spec sheet says 1.7V typical, so 10.8V per string.  I had not 
calculated the brake current yet, but I figured as long as there is enough 
base current to saturate the transistor, there should not be a problem.  
Spehro brings up a good point re: VceSat though, as it is significant 
compared to Vref of the LM317.

Cheers,
-Neil.



On Friday 05 August 2005 02:57 pm, Dwayne Reid scribbled:
> At 12:31 PM 8/5/2005, PicDude wrote:
> >Hi all,
> >
> >I need to supply power to a large number of LED's in a vehicle, where all
> >LED's will be on or off simultaneously, and with 2 brightness levels.
> >
> >  I came up with this circuit, though I'm not sure if it
> >would work properly...
> >         http://www.narwani.org/neil/electronics/Const-2Cur-Ckt.gif
>
> What is the voltage at the top of the LEDs?  In other words, how much base
> current will you have when the brake line goes HI?
>
> If these LEDs have the usual 1.7V forward drop each, your circuit should
> work just fine.  Base resistor value needs to be fairly low or, better yet,
> perhaps consider the use of a logic-level FET instead of the transistor.
>
> dwayne


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