led lighting question

[lp670mike]

Let's say a lamp is the pockets on your jeans and the voltage/current is the coins you put in the pockets

If you go the bank and withdraw $12,   They give you instead $14.40 because that's what they have to give out.... You would be happy, but let's say they gave it to you all in pennies... But your pocket can only hold $12 but it can stretch to maybe hold $13 in pennies maximum... And anything more would start to distort your pocket to the point of saturation or in other words your pocket Tears open and you lose all of your pennies.....

If it were a normal day and you used your pockets as running lamps you would just replace them daily sort of like your boxers... But this day you needed to use your pockets as brake lamps and just when you put your hand in your pocket to take out some brake lamp pennies to use them at the drive through, the pocket rips open and the guy in the lineup behind you at mcdonalds rear ends you....
This is only to find that your pennies and pockets were not correct currency or a real pair of Levi's but something you just stitched together based on random information you interpreted from an online source that suggested you could fill them pockets all day long and never have to worry about them tearing...
Or, let's imagine fitting a bowling ball under the space of your bedroom door.... It can be done but not without some sort of collateral damage.
A little bit of knowledge is a dangerous thing and for warning markers/tail lights it is not to be taken lightly... In canada we have a very strict law about dot approved lamps on vehicles....

Wait.  So what size jeans do I need to make the lights work?

[Digibeam]

Let's say a lamp is the pockets on your jeans and the voltage/current is the coins you put in the pockets

If you go the bank and withdraw $12,   They give you instead $14.40 because that's what they have to give out.... You would be happy, but let's say they gave it to you all in pennies... But your pocket can only hold $12 but it can stretch to maybe hold $13 in pennies maximum... And anything more would start to distort your pocket to the point of saturation or in other words your pocket Tears open and you lose all of your pennies.....

If it were a normal day and you used your pockets as running lamps you would just replace them daily sort of like your boxers... But this day you needed to use your pockets as brake lamps and just when you put your hand in your pocket to take out some brake lamp pennies to use them at the drive through, the pocket rips open and the guy in the lineup behind you at mcdonalds rear ends you....
This is only to find that your pennies and pockets were not correct currency or a real pair of Levi's but something you just stitched together based on random information you interpreted from an online source that suggested you could fill them pockets all day long and never have to worry about them tearing...
Or, let's imagine fitting a bowling ball under the space of your bedroom door.... It can be done but not without some sort of collateral damage.
A little bit of knowledge is a dangerous thing and for warning markers/tail lights it is not to be taken lightly... In canada we have a very strict law about dot approved lamps on vehicles....

[String]

Errr , ok , from what I read in the link and the above info ,  it seems that the info 'mike' was giving was that led's have a 0.5 to 2v range , this would be fine at 12v constant , however a charged battery in good condition would offer something like 12.4v .
The engine running and subsequent alternator output would typically be 13.8 - 14.2v , this therefore makes the complete electrical system that same voltage ... so if I averaged 14v then this would put 12v led's at an overload point .
So looks like you would need two led circuits to achieve the effect you are looking for ..

[SchulzeA]

I'm just posting something that worked for me! So it's factual information.. I'm sorry that you take offense to my help. Apparently your way was the only way?
Feel free to offer machinist info. I may or may not agree, but I won't puff my chest and brag that it's what I do for a living. 
Oh and by the way, I do have my low voltage certification. But I figured Pulse Width Modulation was a bit extreme, so I posted the link.
 

[autopro]

There is nothing wrong with SchulzeA suggestion above?  Beside what does it matter what the alternator output is?  All you need it a 12v source, it doesn't have to come directly from the alternator.

[Digibeam]

And you should believe everything on the internet....
Automobiles alternators do not put out 12v

I guess I should post links to machinists sites with real knowledge rather than ask someone on this site who does this for a living?

[SchulzeA]

http://www.mikesflightdeck.com/led_dimmer/leds_and_dimming.html

[Digibeam]

Tail lights/brake lights have a legal DOT design and output that must have minimum requirements met....
A resistor in line with your LEDs are not really the answer as the running lamps should be constant and a secondary system which would be the brake lamps would be a different intensity this being said, a dual led design is easy to build but the output still has to meet minimum requirements which requires lens and a certification but failing that a quick trip to princess auto will find you in the trailer section where you will find led lamp modules that can be used for building what you need.
They also have the resistance and limiting circuits
And failing that I know a guy that owns an led lighting company...

[farmer]

ok so i have an idea about doing the tail lights on my reventon, however im not too sure if it will work. 

im thinking about getting  some 12v red leds for the parking and brake lights, however i am thinking about running the parking light feed through a resistor to drop the voltage down to make the light about half as bright. the when the brakes are applied a 12volt  would be applied to the led past where the resistor is to give the led full voltage. In theory this  should work but i do not know alot about 12v led lights , one key factor being operational voltages of leds. 

any input would be appreciated   thanks

bryan