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changing LED color question
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changing LED color question

by pencorco on Sun Feb 28, 2010 9:13 pm

I am sorry for anything I ask that sounds too basic or has been answered before. I have searched the forums for my answers and haven't found any that catered to my needs. I have bought the miniPOV kit to make a hand crank style toy. I will be using it to cheer on my daughter at her competitions. These are cheerleading competitions so the lights are usually out and it is dimly lit. Her team color is green. I would like to change the color of the LEDs to green. I understand this might involve also changing the resistors and also possibly changing the battery size or adding another AA to the box to raise the voltage to 4.5V to allow LEDs with a 3.5v forward voltage. Please tell me what I need to do along with links to the needed components. Please speak plainly in your help as I am not an electronics tech. I truly appreciate any help you guys can give. Thank You.
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Re: changing LED color question

by mtbf0 on Mon Mar 01, 2010 9:38 am

according to ohm's law.

(4.5v - 3.5v) / 0.020a = 50ohms.

you know, assuming you want 20ma through the led.
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Re: changing LED color question

by pencorco on Mon Mar 01, 2010 5:25 pm

Thank You mtbf0 for the response. My familiarity of Ohm's law dates back to my NAVY days. Which are quite hazy. Please excuse my ignorance to electrical theory. I have been looking at numerous LED's on multiple sites. But I just can't find green ones that match the parameters of the red ones supplied with the kit. I really am looking for a link to the needed LED's and also links to the new resistors if need be. Also looking for the info on the extra battery. If I could have found a hand-held 360 degree view programmable POV display I would have just bought it. This is my attempt at a picture of what I'm trying to end up with.

Image
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Re: changing LED color question

by pencorco on Wed Mar 03, 2010 5:40 pm

I still really need help changing the color of the LEDs to green. Anyone have time to give me the answers I need. Thanks.
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Re: changing LED color question

by adafruit on Wed Mar 03, 2010 10:38 pm

mtbf gave you the correct answer, why not go with that?

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Re: changing LED color question

by mtbf0 on Wed Mar 03, 2010 11:23 pm

as a rule red leds will have a voltage drop in the neighborhood of 2 volts. green leds will be around 3.4 volts. i reckon this has to do with the chemistry involved. truth is that diodes emitting light is just a happy accident. you use what you can get.

i'll pit my ignorance of electronics against your's any day. just pick some green leds and do the math. supply voltage minus voltage drop all divided by the current you want to put through the led. take the result and pick the next higher value resistor.

you might try sparkfun. their prices are a little high for leds but they do not have a confusing array of possibilities. you'll do better pricewise at digikey, but the number of choices will make your brain bleed.

as for batteries 3 alkaline aa's will give you 4.5 volts. 3 nimh aa's will give you 3.6 volts.
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Re: changing LED color question

by pencorco on Thu Mar 04, 2010 3:14 pm

Thanks guys. I have sat down and found some pieces that might be what I had in mind. Would these pieces work for my purpose?

Battery box: 3xAA
Image

LED: (green) InGaN http://www.theledlight.com/pdf/SS5G4UAEC-45degGREEN.pdf
Reverse Voltage (V) 8
Forward Current(mA) 20
Peak forward Current(mA) 120
Power Dissipation (mW) 45
Operation Temperature Range(¡æ) -40~85
Storage Temperature Range(¡æ) -40~100
Forward Voltage 3.50V
Reverse Current VR=5V 10uA
Luminous Intensity 10000mcd
Peak Wavelength 530nm
Spectral Line Halfwidth 25nm
Viewing Angle 45deg


So...(4.5 - 3.5) / 0.020a = 50ohms....sounds familiar
My only question about this equation is that when I use the internet LED calculator it tells me to use 51ohm resistors. What value resistor should I use? And another question is. When I do the math, should I use the typical values for the LED or should I use the MAX values for forward current and voltage?

Last question. Would these parts, plus the needed resistors, work with the miniPOV?

Thanks a lot for all the help.
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Re: changing LED color question

by mtbf0 on Fri Mar 05, 2010 12:05 am

looks good but the viewing angle on your leds is about 50% greater than the leds in the kit. for a pov you really do want a pretty narrow viewing angle so the dots don't bleed together.

hmmm. but with the leds running around the edge of a circle with a 12 inch radius the leds won't be aimed at the viewer, so a larger viewing angle might be better.

i don't know. forget i said anything.
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