[sdiy] high output LED question - somewhat OT

[MBEDTOM at aol.com]

In a message dated 9/5/2005 5:49:51 PM Central Standard Time, 
jlarryh at iquest.net writes:


> 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.
> 
> 

Larry,

I suggest the following source for LEDs:  
http://www.superbrightleds.com/leds.htm  The RL5-W18030 is awesome at 18 candelas.  You probably have at most, 5 
candelas now.

However, what is the color of the existing LED?  With only 2.4V available, 
that is insufficient for much other than red or orange LEDs.  This is a guess, 
but I would strongly suspect there is a boost power supply there.  Lights that 
use a single battery MUST have a boost power supply to put out enough voltage 
to excite a white LED, assuming the LED is in fact white.  (White LEDs are 
actually blue LEDs with a glop of phosphor on the emitter.  This phosphor is what 
actually emits the white light seen.  That also explains why so many "white" 
LEDs look bluish ... the blue light seeps through the phosphor.)

Googling for schematics pretty much came up with nothing.  (I know you 
already tried that, too!)  It is possible the LED used in there has a built-in 
current limiting resistor.  Using an LED without a built-in resistor would be easy 
so long you know that it has to be there.  Is this light a single battery or 
dual battery unit?  (With a stated 2.4V I would guess two batteries.)  I'm a 
big fan of NiMH batteries and would suggest you also swap out the battery.  
Walmart has 2500MaH "AA" NiMH batteries for around $2 each.  If your light uses 
NiCAD batteries they are probably rated at 800MaH at the most.

Aside from http://www.superbrightleds.com/leds.htm I could also suggest 
http://www.hosfelt.com as an LED source.  I am not impressed by what I have seen 
from Mouser, Digikey, etc.

Peace.
Tom Farrand
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