I suspect I've done the identical project here already, so I might be able 
to make a suggestion or two.

First, the difference between the low and the high current isn't high 
enough. If you need 210 ma to make the running lights visible in daylight 
at high noon, then you need about 3.5 times that current to make the brake 
lights stand out when you step on the brakes.

I think you will find that the dual filament lamp used for running lights 
and brake lights has about the same ratio between the running light and the 
brake light current.

For my project, I used a modified 12 volt to 3.3 volt switching regulator 
from our old cell phone. It had an MC34063 switching regulator in it, which 
is already set up to run on 12 volts and can be configured as a step up or 
step down switching regulator. It is good for 175 ma output at up to 40 
volts WITHOUT using an external transistor.

In my case, I used 4 strings of LED's (only 4 limiting resistors are 
needed, one for each string). I used a  very small mosfet to short out an 
added resistor in series to the voltage feedback pin so that the power 
supply supplies 29 volts for running lights only and around 33 volts for 
brake lights and running lights together.

I'm running 40 ma for running lights only and 150 ma for running and brake 
lights.

The switching regulator runs very warm, but it does run continuously. I'd 
feel a little more comfortable if it was running a little cooler, so I 
might add the transistor to the switching regulator to dissipate the extra 
heat.

I was also wondering about using a photocell to adjust the operating 
current for night time operation......the LED's are brighter than the lamp 
type lights are in the daytime. At night, they are way to bright and could 
be just as visible with much less current.

Enjoy.

Art


and At 02:31 PM 8/5/2005, you 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 also
>want to use a constant-current source for this.  With the matrix I'm planning
>to use for the LEDs (14 parallel strings of 6 series LEDs each), my initial
>high-level calcs are ~210mA normal and 350mA when the brakes are pressed.
>
>To keep the whole thing relatively small and low-cost, the good old generic
>LM317 comes to mind, but the question is how to set up a single LM317 for
>dual current 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
>
>Any ideas?  Is there a better way to achieve this?
>
>
>Cheers,
>-Neil.
>
>
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