William Ruehl wrote:

> I would recommend the LM2940 from National. (www.national.com).  This
> part is a low drop out (LDO) regulator that won't use up tons of power
> in the form of heat like the 7805.

  I'm sorry, but you guys are getting entirely confused as to what LDO
regulators are.  There is as they say, "no such thing as a free lunch"
and translated to regulators, this means that LDO regulators draw *more*
current than an "ordinary" one (10mA nom vs. 3mA nom for a 78L12).

  I refer to the "quiescent" or idle current.  At full load of 1A, the
LM2940 uses "only" 30mA (nominal) unless of course you run it at low
drop-out (less than 3V differential) when it uses *more*!

  This current penalty may be fine for robotics regulating 5V from a 6V
battery, but for an automotive application, the quiescent current of the
LM2940 represents a possible doubling of the battery load even *before*
you switch on the LED.

  You are in fact better off using a plain old 78L05 for the job.  If
you *really* want to save current, you could use a low power regulator.
These are *not* low-dropout.

  Can I tell you a trick?  Put the LED in *series* with the 12V to the
regulator, and have the PIC just switch a 270 ohm load resistor.  The
LED will switch between the minimum 3mA of the regulator quiescent drain
which you cannot avoid anyway (except by using a true LP regulator) and
an 18mA "on" current.

  Forget PWM unless you propose to use an inductor (and commutation
diode).  If you think awhile on the theory, you'll realise it does
nothing.
--
  Cheers,
        Paul B.