On Aug 3, 2005, at 4:48 PM, Matthew Miller wrote:

> Now [EE] was put in the subject for a reason... ;) You must not be  
> aware of
> the paragon of efficiency that is the LM3909. I have LED flashers that  
> run
> for years on one 1.5V D battery!

Efficiency, hah!  More like stinginess.  I was always a bit disappointed
in the results of 3909 circuits.  Since the typical 'flash' was derived
by discharging a 300uF cap through the LED (in series with the battery
to get enough voltage), it was both rather dim and rather short in
duration...

The 3909 came at a time before modern CMOS logic, and before boost
mode switching regulators.  IMHO, there are better ways to accomplish
the same thing now (still, I've been a bit surprised that no one has
put a "low current consumption "flash"" capability in the various white
LED driver chips that are around.  Seems like it would be a trivial
addition, and worth it just to eliminate interest in the 3909...)

Someone has already posted the discreet replacement:

    http://home.cogeco.ca/~rpaisley4/LM3909.html

There are some other 1.5V LED flashers here:

     
http://ourworld.compuserve.com/homepages/Bill_Bowden/ 
page10.htm#15flash.gif

(some dependent on HC logic families seeming to operate down to 1.5V,
which I'm not sure is fair...)

The idea to use a PIC isn't as silly as it sounds, especially for  
something
that will operate continuously.  I can envision a PIC driving a voltage
converter that drives both an LED AND itself.  It might need a button
push to bootstrap itself, but after that it should be pretty easy for  
the
PIC to generate enough 4V or so to power an LED for a tenth of a second
PLUS itself for the second or two till the next flash is supposed to  
occur.
Might be a good entry for one of those contests coming up...  (Mine!)

BillW

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