Flash controller for 12V brake/tail system

Hi all,

I am trying to build a controller to flash an automotive LED brake/
tail light. The light has a 3-wire interface: white is ground, while
black/red select output brightness level. All LEDs are lit,
regardless of brightness. I'm not sure how this is accomplished
internally (the light is a 3-wire black box to me, I can't crack it
open).

During operation, I want the light to alternate between low and high
output modes (frequency/duty TBD...most likely something in the 2 Hz -
5Hz range). The simplest way as I see it is to connect the 'low' wire
to +12V and then have the 'high' wire get switched on/off by a simple
timer (a 555 in astable mode seems right for the job). I haven't
measured the current through the device yet, but I believe that it
will be more than the ~200 ma the 555 output can handle.

I want to avoid a relay if possible...so a power transistor (as a
switch) seems to be the way to go. The designs I have seen show the
(NPN) transisitor emitter connected to ground and the load connected
to the collector...clearly no good for this application since the
switching must be done on the 'positive' side of the LED.

This is a simple problem, so I'm sure it has already been solved.
Does anyone have any ideas (or links to good online resources)?

Cheers,
Dave

.

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