[sdiy] high output LED question - somewhat OT

[simon.oo.o at xs4all.nl]

Hi Larry,

J. Larry Hendry zei:
> I need to draw on the expertise of anyone in the group concerning LEDs.
> OT,
> cause it is not actually a synth project.  But, it is DIY.
>
> I have some of those solar powered walk lights that charge up the
> batteries
> by day and discharge by night.  trouble is, the LED is just not bright
> enough.  I opened it up and found a quite simple circuit using a
> transistor
> to turn on the LED biased by another which keeps it off when there is
> sufficient voltage from the solar cells (which must mean daylight).  The
> battery voltage is abut 2.4 volts fully charged.  And that will delver
> about
> 6 mA into the LED.
>
> The current flow path through the LED has no resistance.  Only the
> junctions
> and internal battery impedance limit the current flow.  So, I really have
> no
> way to increase the current to the LED other than starting over with the
> circuit (higher voltage, more cells, etc.)  I'm not going there.
>
> So, to accomplish my goal of more light, I wish to replace the existing
> LED
> with one that is much more efficient in the amount of light produced for
> that mount of current.  I'm headed off to Mouser to search through LED
> specs
> and see if I can learning anything.  Any suggestions concerning LED
> selection criteria would be greatly appreciated.

Without a resistor to control the current into the LED the light output
will be very dependent on battery voltage, and it is possible that the LED
lifetime will be shortened.

You could use a step-up converter so that you have enough voltage drop for
a series resistor. Alternatively you could use a special LED driver, which
uses a low value current sense resistor, so this solution has a better
power efficiency.

Have a look at (for example)
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2541/ln/




Vriendelijke groet,

Simon Brouwer
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