Re: 1.5v src, LED needs 3v, now what?


SklettTheNewb wrote:
> Hi Chris,
>
> Thanks for the explanation of dual supply, makes sense now.
>
> My power source is turning out to be a troublesome factor. I like
the
> idea of a smaller source(lithium) but I don't know what that would do
> to my cost. I have also thought of using 2 hearing aid batteries to
> get 3v, but I don't know if they will have enough mAh.
>
> I like your suggestion that I post my circuit, here it is:
> http://www.pmddirect.com/temp/comparator_c.gif
>
> (BTW: If anyone has checked out www.pmddirect.com, that is my Dad's
> company, I'm not an engineer for them, thank god!) ;)
>
> So I will explain this circuit, it's a basic sensing circuit that
will
> turn the LED on when the resistance at the sensor get's high enough.
I
> intend to have a flashing circuit that will cause the LED to flash w/
> 1500 ms off for each "flash" (to save power and get attention)
>
> In no way do I think this is a correct circuit, it *works* for me,
but
> I'm sure it could be better/simpler and I would really value any
> suggestions you may have in that regard as well.
>
> In summary, I need a sensing circuit w/ a flashing LED(bright) in as
> small a space as possible utilizing as little power as possible and I
> would like it to be able to flash at the above mentioned rate for 45
> days(if that is possible)
>
> I am REALLY looking forward to any responses :)
>
> Thanks!!
> Steve

You're several years too late, Steve. National Semiconductor used to
make a chip called the LM3909, which did exactly what you're talking
about -- long life flashing of an LED from one 1.5V battery. You could
even set it up so it would trigger on a voltage input.

Take a look at this website, and also take a look at the old LM3909
data*** (.pdf). You should be able to get some ideas from this, and
also construct a circuit practically identical to the LM3909 out of
discrete components.

If you set your LED flasher up for a short enough flash time with long
enough space between flashes, you might be able to get 6 months out of
a AAA battery. However, with the voltage divider for your sensor, I'm
not sure. There are other ways to do that part, though.

Also, even though the LM3909 is obsolete and has been for years, you
still might be able to find a couple or scrounge them from something
else.

Look at this stuff and see. You might also just want to build the
"de-integrated" circuit for yourself for experimenting.

http://home.cogeco.ca/~rpaisley4/LM3909.html
http://www1.jaycar.com.au/images_uploaded/LM3909.PDF

Good luck
Chris

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