Author Topic: 50mA versus 100mA  (Read 7048 times)

Offline tucker

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on: March 04, 2008, 10:27:49 PM
I just completed my IR pen and it does not seem to work.  The specs on this LED are as follows:

5mm INFRA RED EMMITTING LED (KIE-7305-1P) radiant intensity-100mW/sr   1.6V @50mA

I am wondering if this LED is not strong enough as the one recommended by Johnny Lee (Vishay TSAL6400) is 100mA. Any help would be greatly appreciated as I am not good with electronics.  I have this LED hooked up to 2-AA batteries with a 30 ohm resister and a momentary switch.  Thanks for any help!
 



Offline dice

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Reply #1 on: March 05, 2008, 08:38:32 AM
what is the distance from the wiimote to whiteboard?  the 100mA is supposed to be better, but some have used the 50mA and it works, but it depends on what your using it for.

Dice :)



Offline tucker

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Reply #2 on: March 05, 2008, 09:16:32 AM
I am using it for the wii whiteboard.  Currently, I am not projecting using an LCD, and am using my laptop to test.  the Wiimote is is 3-4 feet away from the laptop screen.  I tried to model my setup as close to Johnny Lee's setup.  The light that shines from his pens seem much brighter, when he is doing the whiteboard demo on a table surface.  When I point my IR pen at the wiimote and press the momentary switch, numbers appear on Johnny Lee's calabration program.  The numbers do not appear when you are more than 3 feet away.  Thanks for any suggestions.
Tucker



Offline inio

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Reply #3 on: March 05, 2008, 09:41:27 AM
The chemistry of 940nm LEDs is pretty much constant, so the light output for a given current is approximately proportional across all bands.  You'll see highly varying mW/sr due to different definitions of how they measure that number.  For the whiteboard application, where you're detecting light bouncing off a diffuse surface at very short range from the LED, the angle doesn't matter much, as long as it's fairly small (<45).  What matters is:

1. how much current you're actually pushing through the LED, and
2. that that current is less than the rated absolute maximum, assuming you want the LED to last for more than a few seconds.

Hooking a AA battery up to an LED directly is unlikely to drive the LED at anywhere near it's rated current.  You really should use two batteries and a current-limiting resistor.