In a discussion with someone else, I was asked if all a car's
"auxiliary" lighting could be replaced with LEDs, and/or how to monitor
existing bulbs with a PIC (so you can tell when a turn signal bulb burns
out, etc.)

On the first point, I probably should go look & see if the federal regs.
about car lighting allow tail lights, turn signal bulbs, etc. to BE
LED-type, and how intense they'd have to be (I think I've seen local
buses with LED brake/turn lighting, though), I said I thought that
everything but headlights & Fog/Driving lights could be done in LED's,
nowadays, but would check.

On the second point, obviously you could put a series resistor in line
with the bulbs, swap bulbs to give you 6V bulbs, and monitor the top of
that dropping resistor, through a 1k resistor to the PIC (If it's at 6V
with 1mA temporarily injected through the bulb for turn signal etc.
bulbs, the filaments's good, if not, the filament's DOA.)  If you used a
comparator, could use a much smaller resistor in the ground line & do
basically the same thing (Compare to say a 1/4 Volt reference voltage,
drop 3/8 volt across the resistor.)  Same basic thing should be easier
to do with LED's - though you shouldn't NEED to, they have such a long
MTBF! <G>

  Mark