designing an led tail light circuit for my car, noob needing help

Thread Starter

jcastyo80

Joined Feb 13, 2020
8
Hello everyone. I came across this site in search of some answers. I am designing some led tail lights for my classic car. I know there are a few out there for sale but expensive and don't really like the look. I don't have any formal training or knowledge just what I've picked up these last few weeks.
I would like to see if the schematics below would work.

Here is some info:
It will be laid out on two boards, only because of the way the tail light housing is made.
Will have an LED ring for the Park light signal
VF = 1.7V-2.5V (Would like to run them at 2.1V)
IF = 20mA​
A set of LEDs for the Turn signal
VF = 1.7V-2.5V
IF = 20mA​
A set of LEDs for the Brake signal
VF = 1.7V-2.5V
IF = 20mA​
A set of LEDs for the Reverse signal
VF = 2.8V-3.6V (Would like to run them at 2.8V)
IF = 20mA​
Running each trigger with a 12V regulator to make sure I get constant voltage applied since it could range between 12V-14V. I have marked the desired voltage in red for each cluster of LEDs. I don't know if my understanding of how LED forward currents and resistors work but, I placed resistors to get the desired voltage drop.

Outer pcb
outer pcb.png

Inner pcb
inner pcb.png

So if there is anything I'm missing, don't need or need to change, please I would greatly appreciate it.
Thanks,
JC
 

dl324

Joined Mar 30, 2015
11,282
Welcome to AAC!

Where are you located? The modification you're proposing wouldn't be street legal in the US.
 

Thread Starter

jcastyo80

Joined Feb 13, 2020
8
Thank you!!
Really?! I did not know that. I'm in TX. So do companies get some type of exclusion? I figured if you can buy them, you can install them, so therefore could just make one.:(
 

dl324

Joined Mar 30, 2015
11,282
Really?! I did not know that. I'm in TX. So do companies get some type of exclusion?
Really and no one gets an exemption.

If you look at any lights you can buy, you'll see that they have a DOT approval marking. Color, brightness, blink frequency, durability, and other parameters are regulated.
 

ebeowulf17

Joined Aug 12, 2014
3,274
For future reference, if you want to try LEDs for non-automotive use, you're pretty close to having it right, but not quite.

When a manufacturer lists a range of forward voltages, that doesn't mean that you can safely expect to operate anywhere within that range. It means that the part you get might fall anywhere within that range.

So, taking the example of your park lights, if they happen to arrive with a Vf of 2.1V, then you've done the math perfectly and you're good to go. However, they might not. If they come in with a Vf of 2.5V, which is within their spec, then you've got a circuit which does nothing. It either won't light at all, or may light VERY faintly. This is because the combined forward voltage of 5 in series would be 12.5V, and you only have a 12V supply, so you don't even have enough voltage to guarantee the circuit will operate, much less run at the desired brightness.

If you put 4 in series with possible Vf from 1.7-2.5, you'd have a total Vf of 6.8 to 10V, leaving 5.2 to 2.0V left to drop across the resistor. If you choose a resistor at 270 ohms, you'd get somewhere between 19.3mA and 7.4mA.

As you can see, the variability of Vf values has a huge impact on circuit performance. You can either choose to drop another LED from the string, making the string less efficient, but more consistent, or you can use a current source circuit instead of a voltage source and a series resistor.

For one-off projects, you can design like you did, and then just measure the actual Vf of parts received and adjust your resistor value accordingly. For mass production though, you have to account for the full range of possible Vf values.
 

Thread Starter

jcastyo80

Joined Feb 13, 2020
8
ebeowulf17, thanks for the clarification on LED voltages. I thought the min and max range on the datasheets meant that the min an LED would light up, taking the parking light example, would be say at 1.7V and the max it could run before you burn it out would be 2.5V. So my thinking was I could run them somewhere in the middle (2.1V) so they weren't too bright or too dim and use the 75Ω or 180Ω resisitor, depending on the number of LEDs in the string, to achieve that desired voltage.
 

dl324

Joined Mar 30, 2015
11,282
So my thinking was I could run them somewhere in the middle (2.1V) so they weren't too bright or too dim and use the 75Ω or 180Ω resisitor, depending on the number of LEDs in the string, to achieve that desired voltage.
LEDs are typically sold with a range of forward voltages, though some manufacturers appear to be binning by forward voltage.

You'll get more uniform brightness if you use a current source; manufacturers have been binning by brightness (at a certain current) since LEDs were first made.
 

Thread Starter

jcastyo80

Joined Feb 13, 2020
8
LEDs are typically sold with a range of forward voltages, though some manufacturers appear to be binning by forward voltage.

You'll get more uniform brightness if you use a current source; manufacturers have been binning by brightness (at a certain current) since LEDs were first made.
ebeowulf17 mentioned the term "current source". What exactly is meant by current source as opposed to voltage source?


Thanks
 

MisterBill2

Joined Jan 23, 2018
6,728
Here is a reality check suggestion, which is that you really need to see if all of those LEDs running at 20 milliamps will deliver as much light as you are needing. The higher brightness LEDs used in automotive lighting run at higher currents, sometimes a lot higher currents.
My experience has been that when the LED replacements first arrived they were about twice the price of the standard bulbs, but now they are approaching $10 per bulb. Price gouging indeed!
But the price of things like the 1157 bulb will be less than the price of what is showing in your circuit drawing. Check the automotive section of WalMart, they seemed to have the best prices last summer. BUT that was in Michigan, Texas may be different.
 

Thread Starter

jcastyo80

Joined Feb 13, 2020
8
so bulbs at 70mA would be ideal for automotive lighting? I know I can get an 1157 LED bulb for cheap, but I have halo LED for DRLs on my headlights, which I did purchase from United Pacific. I wanted to brighten up my tail lights and thought of a custom lighting solution to have a halo ring around when park lights are activated and separate turn signal and brake signal lights. For now I'll have to hold off on my idea until I get something figured out I suppose. Thanks for all the responses from everyone, I've learned quite a bit and was definitely way off on my thinking.
 

MisterBill2

Joined Jan 23, 2018
6,728
so bulbs at 70mA would be ideal for automotive lighting? I know I can get an 1157 LED bulb for cheap, but I have halo LED for DRLs on my headlights, which I did purchase from United Pacific. I wanted to brighten up my tail lights and thought of a custom lighting solution to have a halo ring around when park lights are activated and separate turn signal and brake signal lights. For now I'll have to hold off on my idea until I get something figured out I suppose. Thanks for all the responses from everyone, I've learned quite a bit and was definitely way off on my thinking.
There is another post and they mention really bright LEDs drawing 700mA, and so that woukld be quite a bit different.
 
I could care less about SAE and automotive lighting standards because Porsche and Mercedes LED lighting is oddly twice as bright as every car I see, so they have a way to cheat that and why can they blind me? I have been rear-ended accidents twice now.
So I made a new center brakelight for my Toyota. Because it was quite dim Toyota used oddball custom low efficiency LED's and the fixture was 9 LED 5.6W total.
I used 3W bead LED's 45mil die (660nm deep red in array 3x3 in series so ~6V) with the original resistor, on a custom aluminium MCORE PC board from pcbway. It works really good at maybe 9W level although max is 18W. Nothing too crazy for brightness, and the rear-window tint does dim it. I almost doubled the power and extra LED efficiency but it's lower using deep red vs usual orange-red LEDs. My next mod is to use a constant-current buck converter instead of hot resistors. Will post pics later when I get back.
Your circuit is missing input diodes to protect against -ve voltage spikes. I would not use linear regulators because they make so much heat.

edit: I think it is better to use fewer high power LED's instead of many low power ones. I'm not sure how big your fixtures are, they are almost always limited by the original 1157 bulb.
 
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Thread Starter

jcastyo80

Joined Feb 13, 2020
8
I could care less about SAE and automotive lighting standards because Porsche and Mercedes LED lighting is oddly twice as bright as every car I see, so they have a way to cheat that and why can they blind me? I have been rear-ended accidents twice now.
Yeah tell me about it. The Cadillac Escalade headlights are crazy blinding bright!!

Your circuit is missing input diodes to protect against -ve voltage spikes. I would not use linear regulators because they make so much heat.
Diodes, you mean like a 1N4007 diode? What would you recommend in place of the linear regulator?

edit: I think it is better to use fewer high power LED's instead of many low power ones. I'm not sure how big your fixtures are, they are almost always limited by the original 1157 bulb.
Can you please point me toward some high powered LEDs. Yeah its not a very big lens, the housing will hold a 2 1/2" x 10 3/4" board.

Thank you
 
An 1157 incandescent bulb is around 27W (2.1A) and 400 lumens, and 8.3W (0.6A) 38 lumens, in the 1" bulb.

To get 400 lumens using red LED's, which have an efficacy of around 60-75 lm/W, you would need at least 6 high power LED's running at least 5W total. 5W is a lot of heat to get rid of.

King of the hill are high efficiency Cree LEDs like XP-E2 but they are small surface mount and need a special PC board for cooling to run them at rated power. Not easy to work with.
Easiest to work with are chinese "bead LEDs" either 1W or 3W from eBay or Ali, they also come pre-mounted on aluminium PC boards as heatsinks for very cheap.

There are zillions of cheap car LED bulb replacements on eBay and such, and I found 95% of them are garbage. Fake specs for brightness and power, they use many small LED's and run super hot and don't last. It's very hard to just drop something into a car socket and get results beating the incandescent lamp. https://www.superbrightleds.com/cat/tail-brake-turn you can see all kinds of cheezy designs- none of which use decent high-power LEDs.

For the circuit you've drawn, the 7812 regulator is not necessary (and wouldn't work as it loses almost 2V anyway) as the LED's don't care if they are running at 14V or 12V. You calculate the resistor(s) using a 14V supply and hot weather. There would be a 1N4004 in series to block -ve voltage spikes and Toyota puts a 0.47uF cap after it (to GND). The LED lights should operate down to around 9V in case of a weak battery and parked on the road with hazard flashers on. So I would say the limit is 3-4 red LED's in series per string.
 

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Thread Starter

jcastyo80

Joined Feb 13, 2020
8
I have been trying to find ways to go about these lights. I even saw a post by @Wendy about using a resistor, zener and transistor for regulating voltage.

I ended up getting my hands on a Truck Lite super 44 tail light here at work that they get rid of when the LEDs don't fully light anymore. It uses 42 LEDs, the same kind I'm going to use in my project. I don't know if they are the exact ones, but I'm using the Cree piranha style LEDs. Anyhow, it has 3 diodes by the connector, which btw is ground, tail light(park), and brake/turn. It has a total of 12 resistors of which 10 are 91Ω, 1 is 390Ω and 1 what looks like a 160Ω. I don't see anything else behind the board and can't really check since the board is glued and filled with a honey looking resin. I'm not sure this is the best way to go about for making them last.

@prairiemystic what exactly do those 3 parallel resistors do?
 

Wendy

Joined Mar 24, 2008
22,155
I am going to be offline for a little while, My file/print server computer litterally smoked freaking out my room mate somewhat. For about a week I'lll be moving into a new computer.
 

dl324

Joined Mar 30, 2015
11,282
what exactly do those 3 parallel resistors do?
They give the equivalent of a 13ohm 9W resistor. Either a 10W was too expensive, too large, they had a surplus of 39ohm 3W resistors, or ...

Note how the LEDs are connected. You can only do that with matched LEDs. If the tail light light you mentioned does something similar, that might be why some are failing. When LEDs are operated well within their maximum dissipation spec, they last for decades.
 
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