# Sequential LED Taillights

#### bwilliams60

Joined Nov 18, 2012
1,398
Happy New Year to all.
I am looking at building a set of sequential taillamps for a classic car and want to make sure I do things right in terms of selection of components etc so I am going to take this in steps. I am going to start out with the reverse lights and I am replacing a single 1156 bulb with 3 sections of LEDs. They are approximately 1-1/8" x 3-3/4" per section.
Question 1: Would you use SMD, through hole or LED strips? My plan is to design a PCB on Kicad and have the board built once I have a working prototype. I am comfortable with building circuits and soldering so I am looking for some experience here on what components are best to use?
Question 2: Would you hook them up in series or parallel and why? I have seen arguments for both.
Question 3: What would you use for voltage regulation? I suspect it will range from 12.6 VDC to 14.4 VDC. 7812/diodes/zener diode????
Question 4: How many LEDs per section? He wants it brighter than original but I don't want to blind everyone.

@Wendy if you are around, can you chime in on this as I know you are really good at everything LED.

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#### MisterBill2

Joined Jan 23, 2018
8,759
OK, and an interesting set of questions. For the most efficient use of a simple method, all of the LEDs in any set would be in series. The brighter LEDs seem to run about 3.2 volts or just a bit more, so sets of 3 in series will take about ten volts, leaving a reasonable space for the drop in a regulator circuit. If you use several clusters of three in series for each section you will be able to get away with just a series resistor to limit current, because maximum efficiency is not quite so critical with a car electrical system, unless it is an EV. Since the current for each cluster will be about 50ma abd the drop will be at least 2 volts, usually 3 volts, the resistors will need to be half watt sizes.

A previous post asking for the same function I referred to another site that has exactly that circuit for incandescent lights, it should also work for LED arrays. For the actual bulbs, I have used LED replacements from the auto parts section of Wallmart. They are also available at many auto parts stores.

Having all 3 sections come on at once, or possibly in the reverse sequence, inside ti outside, for the stop-light function would be an interesting feature to consider. IT would take a bit of thinking to create that added function.

#### Wendy

Joined Mar 24, 2008
22,431
How many LEDs on the sequence? I assume the backup lights are not sequenced, If they are lay out a plan of what the pattern is. I use M/\$ Paint to draw each step. If you are using high wattage LEDs they will need to be mounted on metal for heat sinking. I have used Super glue for mounting but solder the wires first otherwise the heatsinking will make it almost impossible. You will need a lens over the lights to meet automotive codes, This will dim the lights down. You don't need a voltage regulator but a current regulator, which is easy enough and will also need heat sinking. Parallel when applicable is better as redundancy for what is essentially safety equipment is preferred.

As always my LEDs, 555s, Flashers, and Light Chasers will have most of what you need including schematics.

#### bwilliams60

Joined Nov 18, 2012
1,398
I am trying to focus on one section at a time. Circuit #1 - Reverse Lights. Sequencing will come later.
Thank you for the replies. One says series, the other says parallel and I am leaning towards parallel due to the safety aspect. If one goes out, they all go out doesn't work for me.
If you look at my diagram, the first three on the left are reverse lights. They are non-sequential. I am starting with them and working my way through the circuitry. So for what I want to do, I believe I have asked the proper questions in order.
So question 1 - I think through hole will work. Diffused lens LEDs. I am not sure on how many per section or how many lines in each section. I am thinking 3 rows of 4?
Question 2 - Like I said, I am leaning towards parallel strings of series LEDs due to the variation in LED Vf.
Question 3 - Not sure on voltage regulation however leaning towards zener diode?
Question 4 - Any ideas or suggestions on LEDs to use or size? I know there are 1W superbright but will they be too bright?

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#### Wendy

Joined Mar 24, 2008
22,431
Personally, I think the mechanics (lenses, where to put the circuit) is going to be the hard part. The electronics is pretty basic. You will need LEDs bright enough to see in daylight. If necessary you can add a dimmer circuit for night time. Whatever, they must be visible under all conditions.

As for current limiting, think on of the new LDO (Low Drop Out) 3 terminal regulators similar to the LM317, With a CC (Constant Current) regulator all your worries about Vf just go away. Yes, I would assume 1W LEDs. They also make 1/2 W LEDs.

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#### bwilliams60

Joined Nov 18, 2012
1,398
@Wendy, I think we are thinking along the same lines so far. I was thinking of an LM317 as you mentioned. The lens is a stock lens that comes with the vehicle so I will experiment with LEDs in a dark room with a photocell or LDR to measure resistance values according to light.
1W Super LEDs seems to be the way to go but I want to make sure they are not too bright.
There will be three circuits in total. 1) reverse lighting 2) taillamps 3) brake/turn lights
Do you think I will need an LM317 for each circuit or will one do all three? I am not sure on the limitations. I guess I should grab a datasheet and do some experimenting to see what it can handle.
One quick question about the LM317. If I use this, do I not drop a couple of volts across it before it hits my circuit?

#### MisterBill2

Joined Jan 23, 2018
8,759
I suggested multiple clusters of 3 LEDs in each section. So if any one LED fails you would still have 9 lit, if you used 4 clusters of three LEDs. And it will probably take that many to have adequate intensity for stop and turn service. Each cluster of three LEDs can have it's own series resistor to limit the current, since you want them to light even with the battery down to 12.0 volts. And that series connected cluster of three will need between 9 and ten volts at about 50 ma. Why struggle with a large number of regulators when a simple resistor for each cluster of 3 will work over that voltage range, and be much more reliable?
The reverse lights are different because they will be white light.
And now really the whole thing is very different because that first post said "sequential", and that implies more than one. The statement was replacing one 1156 bulb. I was presuming three separate lenses and the sequential turn lights like the classics. Backup lights were not part of the discussion. Brighter than the original is easy, just by adding more clusters of three in series. And then dropping the voltage to everything for night lighting can reduce the brightness seen in the dark. Simple and easy.

#### bwilliams60

Joined Nov 18, 2012
1,398
My first two lines of my first post spelled out exactly what I am going to do very clearly. I am going to build a set of sequential taillamps and I am starting first of all with my reverse light section. There is nothing sequential about reverse lights so I am only looking at LED selection, best way to place them and how to deliver constant current in the best manner. When I get to the next step which is the tail and brake/stop lamps(sequential), I am sure you are correct but for now, I am on reverse lights. I am doing this in this manner because I am sure there will be someone else that will come on here and want to do the same thing. It will all be laid out on here in a proper order for them to follow. Stay with me, we will get there.

#### MisterBill2

Joined Jan 23, 2018
8,759
OK, but since reverse lights are the least used on most cars, they are often toward the center in the rear.
Reverse lights, like the others, can probably be best done as multiple series strings of three LEDs plus a resistor to set the current. Multiple series clusters will provide the needed redundancy as well as additional light. Many of the LED manufacturers have detailed information on their websites, and I do recommend looking there. They cover both circuits and mounting, as well as the heat sinking that will be needed.

#### bwilliams60

Joined Nov 18, 2012
1,398
I can appreciate what you are saying however I have a question regarding these reverse lights. Lets say I am using three rows and I need 5 lights in each. You say to use a resistor for each string which is common practice. My question comes from the difference in voltage between battery and full charging. That can range from 12.6 VDC to 14.4 VDC and that changes the LED current substantially in that range. Current regulation is done how?
This particular taillight is laid out exactly as I have it drawn in my attachment. Three reverse (white) lights on the inside, and red on the right.

#### MisterBill2

Joined Jan 23, 2018
8,759
I can appreciate what you are saying however I have a question regarding these reverse lights. Lets say I am using three rows and I need 5 lights in each. You say to use a resistor for each string which is common practice. My question comes from the difference in voltage between battery and full charging. That can range from 12.6 VDC to 14.4 VDC and that changes the LED current substantially in that range. Current regulation is done how?
This particular taillight is laid out exactly as I have it drawn in my attachment. Three reverse (white) lights on the inside, and red on the right.
Usually the way to select the current limiting resistance is to pick a number for the max voltage, 14.4 volts, and then determine the voltage drop of the series string when operating a bit below the rated max current. Subtracting that voltage from your selected voltage (14.4 volts) gives you the voltage that must be dropped across the series resistor at maximum system voltage, using Ohm's Law. R= Voltage / current. If the current is in milliamps then the resistance is in Kilohms.
Then in normal operation with a battery voltage of about 13.6 volts the current will be well within intended limits and the light will be bright enough and the LEDs will last a very long time.

#### crutschow

Joined Mar 14, 2008
27,205
Not sure on voltage regulation however leaning towards zener diode?
I would consider a current regulator, so the brightness is independent of battery voltage.
The cheap LM317 can make a tough current regulator with just one resistor.
It's minimum voltage drop is about 3V, so the total LED drop must be about 9V maximum.

If you need a lower dropout voltage (about 1V less), you could use an LM1117.

The LTspice simulation of the two devices in constant-current mode driving 3 LEDs in series is show below.
The 3 (arbitrarily) selected LEDs gave a forward drop of about 3.1V.

The value of R1 and R2 determine the current, which equals 1.25V / R.

As can be seen, for those particular LEDs, the LM317 (yellow trace) drops out at an input voltage of 12.4V (red trace), and the LM1117 (blue trace) drops out an an input voltage of 11.73V.

The regulators can also be used as a switch to turn the LEDs on and off, shown here being done by small MOSFETs M1 and M2 (could also be a small NPN).
When their gate inputs are high (green trace), the output current is off.

#### MisterBill2

Joined Jan 23, 2018
8,759
The number of rgulators will grow quite large because each string will need one. and they will need heat sinks, and a LOT OF WIRING. And while it is certain that the light output will vary a bit, it does not matter, It varies with incandescent lights as well, and it has done that for well over 50 years and it has never been a problem.

#### bwilliams60

Joined Nov 18, 2012
1,398
Interesting discussion and glad I asked because once again, it seems like people who know a lot more about this than me, cannot agree on which method is better.
Although I like the simple resistor method (and I do know how to figure out strings and resistors etc), I also like the idea of better current regulation and as I said earlier, the LM317 looked to be a solution for that. The problem I see is the voltage drop it produces as well as I do not know how many LEDs I can run off one LM317. If I can run say 15 LEDs, this may be okay.
Well I guess maybe the only way to find out is to build both and wee what happens.
@crutschow I appreciate the simulation. Always cool to see things in motion and the expectant result.
I just looked up the LM317 and LM1117 and they both look like they will carry the current for this circuit.

#### crutschow

Joined Mar 14, 2008
27,205
I do not know how many LEDs I can run off one LM317. If I can run say 15 LEDs, this may be okay.
Depends upon the forward voltage drop the the LEDs you use at the desired current.
Certainly it's much less than 15 since red LEDs have a forward drop of around 1.8V each.

#### MisterBill2

Joined Jan 23, 2018
8,759
High brightness LEDs are a bit different than the more normal ones. so it may be that a single series cluster might be 5 or even six, if the drop is less than 2 volts. My experience with the really bright ones is limited to white ones, with a forward drop around 3.2 volts at 50 ma. I know some run a much higher current than that, but I have not seen much written about the colored LEDs in the lighting publications. In addition, it is really better to not run them right at max current, and the additional light output is not worth the pain of much shorter lifetimes. It is not like a race where being the fastest is everything.

#### shortbus

Joined Sep 30, 2009
8,578
but I have not seen much written about the colored LEDs
Why would colored LEDs be used? The lenses are red after all.

Another thing to think about is most LEDs have a pretty narrow light angle, compared to incandescent's.

#### shortbus

Joined Sep 30, 2009
8,578
@bwilliams60 Look at how the OEMs are doing it, your a car guy right? Go to the local car dealer or parts store and ask to look at some replacement lights.

#### MisterBill2

Joined Jan 23, 2018
8,759
Why not use colored LEDs? It would be a whole lot more efficient. Of course the white ones have been developed because that is where the market is. But using a red lens wastes all of the light that is not red. There is no color conversion going on, only color filtering. AND the white LEDs do have a much higher forward voltage drop. Over 3 volts at any useful level. That was my reason for series strings of three: To assure illumination when the battery got a bit below 12.0 volts.

#### crutschow

Joined Mar 14, 2008
27,205
I'm pretty sure that all the automotive red LED tail-lights use red LEDs.
It would be more costly and inefficient to use white LEDs.