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LED Brightness / Resistors / Amp question

Hi Forum

We are planning on having one Arduino Max powering 12photocells and 12 LEDs to fade in and out at various speeds and rates. We are using 10mm white LEDS: http://www.radioshack.com/product/index ... Id=3125355

And we're currently powering everything from the arduino's 5V output. The arduino is powered through USB from the computer. In between the LEDs and the Arduino we're using 220k (2%) Resistors (blue).

Questions:
(1) Will the arduino's 5V power hold up for this or do we need another separate power adaptor for the photocells?
(2) How can we increase the brightness of the LEDs? Can we switch to a lower resistor? ie: 110k?
(3) Besides lowering the resistor what else can we do with those current LEDs to make them brighter? I saw on a tutorial that if you place two 3V batteries (the thin ones) on top of each other to power an LED it'll be brighter than just powering it with a single LED (more batteries more brightness...) What is the electrical theory behind this? Can this be applied to our project? A different kind of power supply?

Sorry I hope this is not too basic of a question... :) any insight or pointers are much appreciated.

Thank you!

Th0m
th0mpics

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Re: LED Brightness / Resistors / Amp question

1) The Arduino power supply plugged into a USB port should be sufficient for 12 of those leds.

2-3) You need to know the forward voltage and current limits of your LED. Formulas and general information about driving LEDs can be found here: http://www.kpsec.freeuk.com/components/led.htm

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Re: LED Brightness / Resistors / Amp question

So in our case we'd need a resistor of 50ohm or higher.
Arduino power supply: 5v (max)
led current: 4v (max)
led amp: 0.020amp

am I doing the math correctly?

Also I noticed that there each resistor also has a wattage (1/4 1/8 and 1/2) and a tolerance number (ie. 5%)

how do I determine those values?

Sorry to keep asking such basic questions ... :)

thank you

Thom
th0mpics

Posts: 5
Joined: Thu Aug 19, 2010 1:10 pm

Re: LED Brightness / Resistors / Amp question

Assuming that the 4v forward voltage for your LEDs is accurate, 50 ohms is correct. I have seen white leds as low as 3.3V, so unless you have a manufacturer's spec, you might want to use the more conservative value for your calculations.

One watt is equal to 1 volt@1 amp. Since you will have 1 volt drop across your resistor @ 0.02 amps, you will need a resistor capable of dissipating 0.02 watts. The 1/8 watt version will be more than enough and the tolerance is not critical.

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Joined: Sat Feb 07, 2009 10:11 am

Re: LED Brightness / Resistors / Amp question

I see. Thank you for confirming my math... :)

So the tolerance and the wattage don't have any influence on the brightness then, correct? A higher tolerance or a higher wattage for example won't make things dimmer? (sorry just trying to get the most out of our current LEDs... :))

Thank you

Th0m
th0mpics

Posts: 5
Joined: Thu Aug 19, 2010 1:10 pm

Re: LED Brightness / Resistors / Amp question

arduwino wrote:Assuming that the 4v forward voltage for your LEDs is accurate, 50 ohms is correct. I have seen white leds as low as 3.3V, so unless you have a manufacturer's spec, you might want to use the more conservative value for your calculations.

One watt is equal to 1 volt@1 amp. Since you will have 1 volt drop across your resistor @ 0.02 amps, you will need a resistor capable of dissipating 0.02 watts. The 1/8 watt version will be more than enough and the tolerance is not critical.

I've never seen an LED with a Vf rating of 4 volts when driven at 20 mA.

Typical is around 3.2-3.6 volts at 20 milliamps for blue/true green/white.

Human vision is logarithmic, so dropping brightness a bit will be difficult to see unless directly compared to another similar light source, so I would err on the side of caution and assume the worst-case Vf (around 3.2 at 20 mA for blue/true green/white)
Entropy

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Location: Owego, NY USA

Re: LED Brightness / Resistors / Amp question

"So the tolerance and the wattage don't have any influence on the brightness then, correct?"

The only resistor parameter that will influence brightness is the actual resistance. Rated wattage will have no effect at all. Tolerance refers to the statistical variability of the actual resistance value, so indirectly it can have an effect if your sample deviates far enough from the norm.

But, as entropy says, the human vision response is logarithmic, so a 5% difference in resistance will not be visible.