# Low Intensity LEDs



## Oubadah (Jul 15, 2010)

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## AnAppleSnail (Jul 15, 2010)

LED brightness is defined by the LED's output at whatever current you give it. Less current = less light. The car isn't 12v though, while it's on it'll be around 14v. You can use a resistor to set the current going through.

Vtotal = 14v
Vf (forward voltage of the LED setup) = Vf. You'll have to look this up.
Vr (Voltage across the resistor) = Vr. You'll calculate this.
I (current) is the same in the resistor and through the LED string. If the LEDs are in series they each see the same current.

Vtotal = Vf + Vr
Vtotal - Vf = Vr

Current at the resistor is given by V=IR. So you know the Vr that you want and a current that you want to run the LEDs at.

14v = Vf + Vr
Vr = I*R
R is unknown
Vr/I = R
14v = Vf + Vr/I

Slight changes in the resistance you pick will have large effects on the LED current, which in turn has a large effect both on the LEDs forward voltage and some effect on the brightness. It gets complicated. Start by measuring the voltage while your car's on.


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## JohnR66 (Jul 15, 2010)

You my find the dashboard graphics and such will soak up a lot of light. I would run some tests before going with dimmer LEDs.

Avoid The cheap eBay so called high brightness LEDs. They have woefully short lives, brightness is overrated and color is too blue is (if you're talking about white LEDs).


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## spencer (Jul 15, 2010)

Instead of changing the LED change the resistor. Far cheaper and probably equally as difficult.


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## Oznog (Jul 18, 2010)

Pay attention to distribution. Highly focused LEDs are popular and in high demand. They produce a narrow, bright beam though, and you may end up with the backlight just creating a bright disk about the size of the LED (or even smaller) on the panel it's supposed to be shining through. Lambertian may work better.


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## Oubadah (Jul 18, 2010)

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## Ken_McE (Jul 18, 2010)

Oubadah said:


> Is there any way to dim these leds without altering the colour?



Sand them.


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## Oubadah (Jul 18, 2010)

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## Noctilucent (Jul 18, 2010)

Have you tried using black tape? I recall that being a popular option to dim the blue LEDs on computers a few years ago. I suspect that may tint the color slightly greenish as well but I have never tried.

If you don't mind spending some money, you could try sourcing a neutral density filter. These come in rigid glasses and plastics, flexible films (termed 'gels'), and liquids. They're used in photography and filming to 'stop down' the exposure of the film/sensor / reduce a light's contribution and are, as the name implies, neutral in color tone.

Another option to 'dim' the LEDs would be to use PWM, but that requires some active components.


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## usLEDsupply (Jul 18, 2010)

just get a cheap PWM dimmer then you can adjust them to whatever brightness you want, or i think most cars still have dimmer switches built in so as long as all your lights are the same you should be able to dimm them enough with that.
if you are having trouble with them being too much of a spot then try some SMD Led Modules as they have a nice wide angle and for $3 you can get 3 of them in a self stick module already setup for 12v in any color or RGB (you will defiantly need to dimm them down maybe to 15% as they are really bright) i used 3 of them on a lawn tractor for ground effects and they were bright enough you could mow with them.


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## Oznog (Jul 19, 2010)

Oubadah said:


> Changing the resistors is just reducing the voltage? When I run these LEDs of a power pack, and turn down the voltage, the LEDs don't just change in intensity, but also in colour. They go from a rich spark blue to a paler greenish blue, which is what I was hoping to avoid. I'd previously just been applying coats of white enamel paint to the lenses, and this was working well to dim them, except with the third coat I began to notice the light from the LEDs going greenish.
> 
> Is there any way to dim these leds without altering the colour?



I'm doubting this. The change in color temp is not that great. And I've never even seen a white LED that I would choose to describe as "greenish" blue.

I suspect more that your eyes and mind interpreted the dimmer light differently. The human eye actually modifies its color sensitivity under different lighting conditions. 

PWM will allow you to drive current at the full rating but the average light will be reduced. But, I doubt you'll find it as easy to pull off.

Even if you DID try a white LED which went "greenish" at low current, it doesn't mean all whites do. _None_ of the ones I've run at reduced current have. So, just try another one, I doubt you'll actually run into an unacceptable color temp from that.


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## Oubadah (Jul 19, 2010)

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## Oubadah (Jul 19, 2010)

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## usLEDsupply (Jul 19, 2010)

if you can't find the right resistor i have a 1 channel PWM slide dimmer that you could have for $8
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## Oznog (Jul 21, 2010)

Oubadah said:


> It wasn't a white led that went greenish, it was a blue one. But I think you're right, and that it was a trick of my imagination.


Gotta be. Because the color LEDs work on a fixed bandgap that sets the color. It shifts SLIGHTLY with temp, IIRC. But power won't make a noticeable difference in color.

A resistor will lower the CURRENT. The LED voltage will not change much and the voltage is not really the factor directly influencing LED intensity, the current is.


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