> Of course 4 cells is reasonable in this case since it is an add-on 
> to your
> existing device. So since you don't need super efficiency, do a 
> sepic
> regulator. By adding one more inductor and a capacitor to this 
> circuit:
> http://bobblick.com/3cell/
> you could do it very cheaply. Voltage regulation is not great, you 
> could
> either use a zener and waste a lot of current or add an LM431 for 
> the
> reference.

Yes. I mentioned a SEPIC as one of the options. The two inductors is 
annoying but not necessarily a killer. And it needs a semi formal 
controller, unlike some options.

You could run a SEPIC entirely open loop, designed to make enough 
power and voltage and zener shunt the excess when needed. Mayhaps open 
loop except with basic regulation turning it on and off. Aim 
throughout is cheapness. Not a lot of power involved - probably about 
15 mW ! -)

I wonder if CUK has anything to add here.



        RM

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