Matching brightness between different length of LEDS (newbie)

[migee]

Hi all,

I have a bit of electronics background but not much with circuitry. I'm trying to make a board of LEDs and came accross a snag in this experimental LED automotive project.

I'm working with a voltage of 14v and LEDS with a forward voltage drop of 1.9 and the LEDs take 18mA. I have a line of 7 and a line of 6 elsewhere.

Line of 7 gets a 39ohm resistor and line of 6 gets a 150ohm resistor to limit the current. Now all is good, but I have two inputs, one for bright and one for low (the above being for the bright mode). I experimented, then put a 560ohm resistor on the set of 7 for when they should be dark. Now the problem is understanding how to calculate a resistor that will work on the set of 6 that will match the intensity of the 7.

I figure....

7(led) * 1.9 (forward v) = 13.3 (.7 left over)
.7/.018 = 39ohm

So I tried calculating what the 560 is actually doing -- is this right?

.7/A = 560
A = .00125

I know that my target amperage is .00125 on the set of 6 for low mode.

So I calculate for 6:

6x1.9=11.4 (2.6 leftover)
2.6/.00125 = 2080Ω

Why when I use the 2080 on the row of 6 it seems much less bright than the row of 7 using a 560Ω resistor? Where am I off in my calculations?

 

[MikeMl]

Read this on **broken link removed**

 

[migee]

I assume this is a warning not to use LEDs for road lighting? Thanks but not the information I am looking for. FYI I am not swapping out either my headlamps or my brake lights, merely adding some leds to compliment them -- hardly much different than adding halos for headlights. I also know the highway act well enough to keep this completely legal.

Thanks for the warning but could I get help on my logic posted please?

 

[MikeMl]

I assume this is a warning not to use LEDs for road lighting? Thanks but not the information I am looking for. FYI I am not swapping out either my headlamps or my brake lights, merely adding some leds to compliment them -- hardly much different than adding halos for headlights. I also know the highway act well enough to keep this completely legal.

Thanks for the warning but could I get help on my logic posted please?

You assume Wrong!
I wasn't making any statement about the legality of your lighting; I was trying to explain that using your eyeball to compare apparent brightness is purely subjective. In other words, put a damn pot in series with your LEDs, and diddle it until you are happy, provided you do not exceed the maximum current through the LEDs.

 

[migee]

Haha, hey thanks. Don't have a pot here but I can pick one up or just mess around. I thought there'd be a surefire way of calculating it. I can 100% tell there is a brightness difference as the leds are right next to eachother and its a day and night difference. I'll give that document another look. I tried to get through it but I think its just too late at night to understand it fully in the way they speak of it.

 

[audioguru]

I hope your LEDs are not bright enough to blind oncoming drivers.
In Canada cars have daytime running lights. Most cars use front turn signal lights or dimmed low beams except Chrysler.
Chrysler cars (and their Jeep trucks) use HIGH BEAMS at FULL BLAST to blind all oncoming drivers. It is really bad when it is cloudy or raining. Stupid.

 

[migee]

I'm taking it very carefully to not add much light. The LEDS are going into the taillight housings (as rings), and the rear panel of the lights are patterned to diffuse most of the light among it. It will be somewhat subtle, just looking for away to make the lighting appear more modern, while still using the halogen. Also it has the benefit of lighting up much faster as opposed to the bulb.

I'll see if I can devise a way to confirm the overall brightness before and after by some technique of measurement