Brightness of LED Backlight

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jeff009
 
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Brightness of LED Backlight

Post by jeff009 »

I bought a large LED Backlight (https://www.adafruit.com/products/1621) but the brightness seems to be quite dimmer than what's in the video
When I tested it out, I hooked it up to a 220 Ohm resistor, into the 3V out on an Arduino Uno.

1. Does the LED backlight need a resistor? It's an LED so I expected it to need one, yet in the product video it appears to be hooked directly up to a battery.
2. If it requires a resistor (as I suspect it does), what resistance should the resistor be?
3. Is the 3V on an Arduino Uno enough power, or do I specifically need a 3V 20mA power supply?

Thanks,

Jeff

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

According to the data sheet, a resistor is required: https://www.adafruit.com/datasheets/KWB-R8445W-1W.pdf
However the forward voltage of the led is in the 3.3v range (2.8v - 3.6v - 3.3v typical)

The standard LED resistor calculation is:

R = (Vs - Vf) / If

So if you are powering from 3.3v source, the resistor calculation would be:

R = (3.3v - 3.3v) / 20mA = 0 ohms.

In other words, no resistor is required when powering from 3.3v

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jeff009
 
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Re: Brightness of LED Backlight

Post by jeff009 »

Can it be powered by a greater voltage then, like 5V or 9V? (and in that case it will need a resistor) Or is the input voltage limited to up to a max of 4V?

Thanks,

Jeff

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jeff009
 
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Re: Brightness of LED Backlight

Post by jeff009 »

Also, I just tried Arduino Uno 3V -> LED (w/o a resistor) and the LED still seems quite muted compared to what's in the video (wouldn't call it bright by any means).

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

The standard LED resistor calculation is:

R = (Vs - Vf) / If

From the data sheet, 'Vf' is 3.3v and 'If' should be 20mA. So, for a Vs of 5v, the calculation is:

(5v - 3.3v) / 20mA = 85 ohms.

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greg_corson
 
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Re: Brightness of LED Backlight

Post by greg_corson »

Maybe a silly question, but the ESP8266 (I have the Feather Huzzah version) says it's GPIOs can handle 12ma and is running at 3.3v

So can I just drive one of these off a ESP8266 GPIO pin with no resistor?

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

Since there is nothing to limit the current drawn by the led to 12mA, you could overload the GPIO pin. A 10 ohm resistor should keep you safe.

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greg_corson
 
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Re: Brightness of LED Backlight

Post by greg_corson »

Thanks, just wondering how you calculate that? It doesn't fit in the formula you gave earlier.

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

That formula is somewhat oversimplified, since it doesn't take into account the fact that the Vf of the led changes with current and temperature. You also have to consider that there is some variation from sample to sample and there is a fairly wide range of Vf on the spec sheet. With no resistance at all, you have no check on the current. The 10 ohms gives you a bit of a safety factor.

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greg_corson
 
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Re: Brightness of LED Backlight

Post by greg_corson »

Two dumb questions...in the product video they connect this to an AA battery pack and to a coin cell without a resistor. Would one of your 500mah lithium batteries be ok too or will those deliver too much current without a resistor?

Is the white plastic on either side of the thing a plastic protector that's meant to be removed? Or is it intended to stay on as a diffuser?

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

in the product video they connect this to an AA battery pack and to a coin cell without a resistor.
Alkaline cells and coin cells have a lot of internal resistance which limits the current they can deliver. Lithium cells do not, so you would risk burning it out.
The white plastic is designed for maximum diffusion.

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samuelguyer
 
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Re: Brightness of LED Backlight

Post by samuelguyer »

Hi! I have a similar question about the backlight module, so I figured I'd post it here rather than start a new thread...

Is it possible to cut the acrylic? I'd like to make a different shape, and maybe even hook it up to a different LED. Does that seem reasonable?

Thanks!

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adafruit_support_bill
 
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Re: Brightness of LED Backlight

Post by adafruit_support_bill »

You can cut it. Just be careful cutting near the LED source. I haven't experimented with using other LED sources, but I would expect it to work. A little bit of clear epoxy to mount it to the edge should help ensure good light transmission.

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samuelguyer
 
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Re: Brightness of LED Backlight

Post by samuelguyer »

I got a couple of these in the mail the other day. They are quite nice, and you can cut them pretty easily. I don't think there is anything particularly special about the acrylic. I tore one apart just to see how it's made. The acrylic is etched on the back with a pattern of dots that catch and scatter the LED light. The back and sides are covered with a reflective material that only allows light to exit from the face. The face is covered with a semi-opaque plastic sheet that diffuses and softens the light.

I tried cutting out the existing LED and replacing it. It was a bit of a pain, but it worked.

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natechang
 
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Re: Brightness of LED Backlight

Post by natechang »

I am also running into a problem with the brightness of the LED Backlight. I purchased the Large LED Backlight and am using two AA batteries directly plugged in, like how Lady Ada does in the video. However, my backlight is not bright at all. In fact, I had trouble figuring out whether it was on or not. Is there something I can do to make it brighter, like in the video? In the video, the backlight seems to be very bright, and exactly what I was looking for.

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