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LED Tail Lamps

Discussion in 'LEDs and Optoelectronics' started by sall, Feb 20, 2010.

  1. sall

    sall

    51
    0
    Jan 29, 2010
    I would like to retro-fit my tail lamp assemblies with LEDs. I am not looking to do sequential tail lamps or anything like that.

    I have a typical GM setup two dual filament bulbs in each assembly. Here are the schematics for reference.

    [​IMG]

    [​IMG]

    [​IMG]

    I am confident in picking a resistor size and how I will wire up the LEDs in the array. What puzzles me is how do I get the LEDs to have two intensities? One for tail lamps and two for brakes. Also how is this going to work for turn signals?

    I would like to wire up two round PCBs in each assembly. The assembly looks as such:

    [​IMG]

    PCB on the inside would be slightly larger than the outside.


    See nothing crazy. I searched the net but I found no conclusive DIY projects with details. I apologize if this has been discussed before. Thanks in advance!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Any particular reason why you don't just purchase the LED globe replacements?

    Depending on your car, you may activate whatever warning it has for a blown taillight though (as the LEDs don't draw "enough" current).

    Oddly enough though, in Australia, replacing the original bulb with a LED "bulb" may make your care *more* in breach of the technicalities of the licensing conditions than would a blown bulb. (that's more an academic point than a warning)
     
  3. sall

    sall

    51
    0
    Jan 29, 2010
    I was not happy with the PnP LEDs I purchased. Plus I would like to define the shape a little more myself. Which is why I would like to use 2 different sized round pcbs in each assembly.

    My vehicle has no warning indicator for burnt out bulbs ('96 Olds Aurora). I believe that problem can be solved by a load resistor if was the case.

    No local or state wide restrictions on LEDs as long as they operate in the same manner as normal filament bulb. Now HID headlamps etc, are a different story.

    Thanks!
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    OK, I would figure out how many LEDs I required. You might choose high brightness red (or white) LEDs or high power red (or white) LEDs,

    My personal preference would be for high power red LEDs, but it seems you would prefer otherwise.

    Once you know that, you determine the best series/parallel arrangement to put them in for the voltage available.

    I would tend to disfavour using a resistor as a current limit as the voltage in the car varies considerably which would lead to variations in brightness (possibly able to be confused with brakes going on/off). The more LEDs you place in series the worse this becomes.

    I would probably use a simple 2 transistor constant current circuit as you can probably arrange it to dissipate only a relatively small amount of heat (and certainly less than the bulb). It won't be perfect, but it will be way better than a simple resistor.
    http://www.elecfree.com/electronic/constant-current-source-led-drive/ Series/parallel wiring of diodes will require resistors to ensure current is reasonably well shared -- I would only advise this if you use relatively low power LEDs. For High Power LEDs, I would advise one current regulator per leg.

    Simply make a board of appropriate size and shape, lay out the LEDs as you desire, place the current regulator in a convenient blank section of board.

    Remember to de-rate components as required due to the potential for high operating temperatures.
     
  5. sall

    sall

    51
    0
    Jan 29, 2010
    Alright thanks for the input!

    So, I would use one constant current source led driver for each LED array, correct?

    This will achieve the dual intensities and maintain proper turn signals?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Yeah, you would have (at least) 2 for each lamp. One for the taillight and another for the stop light. (yeah, one for each array)

    I'm not sure what the relationship is between the taillight and stop light wattages for the original bulbs, but it makes sense to keep the number of leds and the total current through each array in the same proportions.

    e.g. if they were 10W and 20W, then you should have (say) 30 and 60 LEDs. The total current for the larger array will be twice that of the smaller array to maintain the same current through each diode.
     
  7. sall

    sall

    51
    0
    Jan 29, 2010
    I took a good look at the constant current driver and I am not seeing how this is going to work with needing two distinct brightnesses to mimic the dual filament bulbs.

    Still new to this so bare with a newb, please.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    You have 2 distinct circuits.

    One has (say) 10 LEDs with the constant current source set to 100mA (5 strings of 2 diodes each running at 20 mA)

    The other has 20 LEDs with its constant current source set to 200mA (10 strings of 2 diodes each running at 20 mA)

    The input shown doe that constant current source is tied directly to the +12V end of the circuit so each set of LEDs and its associated current source have 2 connections (like a filament bulb)

    I'm not going to be able to respond for 2 weeks as I'm off overseas. Unless you're in Hong Kong, Taiwan, or the very southern parts of China, You'll have to get someone else to assist you for a while. I'm sure someone will pitch in. :)
     
  9. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    I hope you get a real nice trip steve!
    I believe sall is thinking of treating the whole LED array as one and modulating the current through it. Some color shift may be noticed that way.
    Anyway, it can be done using two constant current sources in parallell, one at 5mA and the other at 21mA, driving the same string of LED's.
    I'd guess you could get away with using as much as 5 red LED's in series in each string, depending on their Vf. Are your tail/stop/turn signals all red btw.?
     
  10. sall

    sall

    51
    0
    Jan 29, 2010
    Yes, the tail/stop/turn signals are all red.

    Two bulbs in each assembly. Both are dual filament.
     
  11. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Ok. How then would you want it? One "skinny" pattern for tail-lights with one 4 times fatter pattern for stop/turn, or would you like just one full pattern changing brightness?
    The light output change will be the same for both and they will look the same from a long distance, but the former will look different up closer.
    I believe tests have shown the former to be safer in that the pattern change is easier to recognize as a signal.
     
  12. sall

    sall

    51
    0
    Jan 29, 2010
    I like the idea of the former.

    I am still up in the air about what LEDs to use. High power or high brightness? Size? Any recommendations? I need them to be reliable for obvious reasons, but I also need to be cost effective.

    Also, when brain storming I thought two separate round PCBs in each assembly to simulate the two bulbs. The PCB on inside would house a slightly larger LED array than the outside PCB.

    What would you suggest? Two round PCB's in each assembly in above fashion or one PCB with two LED arrays?

    The assembly lens is actually diffused. So, thinking about that I figure it will be rather difficult to get a defined array of lights, especially two separate arrays, to actually appear separate. The only way I could think of was to have LEDs extremely close to the lens. Even then they may not be defined. Plus to add I have them tinted pretty dark as you can see from the picture on page one.
     
    Last edited: Feb 21, 2010
  13. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    It's a little hard for me to give recommendations as to the actual layout and hardware to use. You don't neccessarily need to mount them on PCB's, and you can make pretty much any pattern you like. I was thinking about an outer rim for the tail LED's and filling the central area with stop/turn LED's. You'll need 4 times as many LED's for the stop/turn pattern as for the tail pattern. I'd have suggested using high brightness 5mm's, but I don't think it'll look too cool being behind the original lens.
    You'll just have to look around on the 'net to pick up ideas. There are many people that have done this and presented the results.
    Ah, tint, yes, they were so dark I couldn't make out anything about them.. Not sure how to deal with this and how it'll impact the brightness..
     
  14. sall

    sall

    51
    0
    Jan 29, 2010
    Here is a picture of what the assembly looks like lit up with one bulb:

    [​IMG]

    The horizontal lines are there from factory.


    Tail Lamp Assembly Disassembled:
    [​IMG]

    Does not appear the diffuser comes out. I don't know 100% sure yet. This picture was taken by a forum member at ACNA when we were brain storming this idea awhile back.

    Also what else can I use to mount the LEDs besides PCBs? Will a piece of plastic cut to fit exactly inside the lens work?

    I would like to use one of two options:

    Horizontal rows of LEDs that alternate each line. This would kind of flow with horizontal lines from factory. These rows of LEDs would be the tail lamps and they would operate at 1/4-1/2 brightness. The other alternating horizontal strings of LEDS would illuminate when brakes/turn signal is applied. I would like for these lamps to operate at full brightness. Is there a way to have the tails dimmed at 1/4-1/2 and then full brightness with the rest of the LEDs strings when brakes/turn signal are applied?

    Other option is the entire array of LEDs lit at 1/4-1/2 brightness with tail lamps and the entire array light to full brightness when brakes/turn signal are applied.


    These are the two I have thought of and think would look good. TIA!
     
    Last edited: Feb 22, 2010
  15. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Yes you can cut a piece of (black) plastic to fit, drill holes for the LED's, press-fit or glue them in place, and then bend & solder their legs together in a rat's-nest fashion. The constant-current drivers could be put on small PCB's/perf-boards wired to the LED's. Yes, it'll be possible to combine both modulating methods like in your first option.
     
  16. sall

    sall

    51
    0
    Jan 29, 2010
    Alright, which method would be the simplest of the two I described?

    I would like to start up with something less difficult and work my way to something more difficult. No other way to learn I suppose.

    I have a good understanding of how the constant current drivers works, but not sure how to implement in dual brightness/modulating fashion.

    Do you also have a preferred place to purchase high brightness 5mm LEDs. What specs should I be looking for?
     
    Last edited: Feb 22, 2010
  17. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    I've never done a taillamp mod myself so I'm not really qualified to say how it's best done, but making such a PCB would be a pain in the .... and wouldn't neccessarily add to the stability or reliability so I'd rather choose the simpler rats-nest way.
    A dual current driver can be implemented either by parallelling two separate drivers or by adding an extra resistor+transistor to the current-determining resistor. I can make a drawing for you later.
    LED's are often specified having so & so many mcd, + having a beamspread angle. This is the clue to the power it has; if you use the same chip and narrow the angle you increase the mcd. So you want a LED with as much mcd as possible combined with a wide beam. Time to dig out your old trigonometry book.. ;)
    This here place has a marketplace and also a great forum with loads of useful info on LED's.
    Keep in mind also to look for LED's with the right red color (wavelength [nm]) as there are several varieties, some being more orange than red.
     
  18. sall

    sall

    51
    0
    Jan 29, 2010
    I would appreciate in you drawing me a circuit, Resqueline.

    I am going to go with the LEDs mounted on an appropriately shaped plastic insert. The constant-current source drivers will be enclosed in a project box and in the trunk under the liner.

    I am more than satisfied with using two constant current drivers. One for each array. Horizontal alternating lines as tails at 5mA and remian at 5mA. The other alternating horizontal lines will be the stop/turn at 20mA. Which I am more than capable of doing.

    I am curious though as how to parallel two separate drivers or the extra resistor transistor you mentioned.
     
  19. sall

    sall

    51
    0
    Jan 29, 2010
    Resqueline,

    In my search today I found a TLE4241GM. It seems this does exactly what I want with very little additions.

    Here is a link to the data sheet.

    Check that out and let me know what you think. I don't know if the fact that this is SMD if i will have trouble with solder joints, etc. Although, I could use a little further explanation of how this is hooked up because i don't quite understand the data sheet diagrams. Again, bare with a beginner please.
     
    Last edited: Feb 24, 2010
  20. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    I must admit that IC had me a little confused. From the datasheet I'm not easily able to make out exactly how it works & is controlled. Apart from that small snag it seems to be ideal for the purpose, and it's not so small that you'll have problems dealing with it.

    Anyway, here's the old-fashioned ways as promised. You'll need one such circuit for every 5 LED. (No wonder people often resort to using only resistors.)
     

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