LED Brake Lights

[DRACO A5OG]

Oh My Goodness, this will totally update our old girls.

Wow, you an artist

[Jonny H]

Spoke, you are on a roll with these and they look great.

From one EE to another though I would strongly advise that you do look into a proper LED driver IC for these. Not only will you get a constant current drive which will give a more stable light, these also protect from transients and reverse voltage.

The issues you are likely to encounter are failures to do with switching inductive loads elsewhere in the car (electric motors, fuel pump, blower etc) and from external systems (jump starting and booster starting). These have the effect of producing high transient voltages. Battery boosters are notorious for momentarily reversing the polarity of the voltage.

About the worst thing that can happen is the battery becoming disconnected when the alternator is charging it. An unlikely event perhaps, but that will typically put a 600V spike of 50J or more on the 12V line.

These are the details that will make your lights not just great looking, but long lasting.

[spoke]

Jonny,

Thanks for the insight on automotive transients. When one considers the transients in ISO 7637-2 it's amazing that any electronics survives in the automobile. I worked for a time at Amplifier Research supporting automotive transient test equipment. My favorite pulse is pulse 5, the load dump or as Toyota calls it: "Giant pulse".

I've gone full circle on the use of an LED driver. The first boards I did used a boost converter. I have them on my 914 now for the brake lights. The good about the boost was very low power dissipation. The bad is transients on the power leads goes straight to the LEDs. A buck converter would be better suited but most buck LED drivers are high side current sense and are very hard to dim without using PWM. With PWM dimming, the circuit is getting more complicated than I would like.

For linear LED drivers, too much power is dissipated in the LED driver and it would get too hot. For example, the 911 LED boards have 12 strings of 4 LEDs running about 20ma on brakes. With 2V per LED, the LED string drops 8V and the control would drop 6V. Power in one driver would be 12 x 20ma x 6V = 1.44W. This part would get very hot.

What I do is use resistors to limit current in each string which dissipate the power uniformly across the PCB. Given the tight operating voltage from about 12.5V to 14.5 the light output of the LEDs is pretty consistent.

I test each board design for temperature rise by running the boards like brake boards with the brake lights on for hours. The 911 brake LED board temperature rises about 20C over ambient which is very good. I've tested the board design for days straight with just the running lights on. Temperature rise with running lights is less than 5C. I check for hot spots with an infrared temperature probe.

The boards do have series diodes to protect against reverse voltage up to 75V. For boards with active components like comparators or FETs, bidirectional TVS's are added.

[Jonny H]

Yes, it was the giant load dump I was referring to! It is good to see that you have considered these circumstances. The real world is a very different place to the bench.

We've been working on a CDI replacement for the last couple of years and the transients are horrendous. Not easy putting a 3.3v micro controller in the same box as a 350V inverter!

There are plenty of products out there that are rushed to market but fail prematurely. The lack of consideration in the design combined with the drive for low cost products is just a race to the bottom. Where has the quality gone?

Good luck with the light units.

[spoke]

Like NFL officials routinely say, "after further review", I've added another row of white LEDs for a more intense visual footprint. I've just finished laying out and routing the board.

This one will have 24 white and 48 amber LEDs. The operation is similar to the front turnsignal LED board that I did for the 914. That one had 21 white and 48 amber LEDs. It will have blanking of the white LEDs when the amber turn signals are active like the 914 board show in the clip in post 147 http://forums.pelicanparts.com/8743977-post147.html on the previous page.

The board has the capability to have all amber LEDs for those with amber lenses. I'll probably run 3 sets to make sure everything works. Should have them in 3-4 weeks.

[heimtun]

Following.

[Neel]

I'll be interested in the these also.. Will we need to change flashers or is the resistance built into the boards??

[TheSt|G]

Any chance you could work in some type of "diffuser" for the LEDs? I love the brightness, the plug/play nature, safety, and the lack of a need for a good reflector, but I really don't want the look of "aftermarket" LEDs.


As an example, the right looks massively better to me than the left:

[spoke]

Originally Posted by Neel

I'll be interested in the these also.. Will we need to change flashers or is the resistance built into the boards??

I do put a bit of resistance on the boards but this is just to help the LED-compatible flashers work. The resistors are not at all what is needed to fool the standard 911 flasher. I put about 250-500 ohms of resistance on the boards. This is about 1/2W. To fool the standard flasher, one has to drain about 18-20W per turnsignal; front and rear. With these boards front and rear, we're replacing about 20W+20W=40W with about 9W total.

Best bet is to replace the standard 911 flasher with an LED-compatible flasher. The one I used is EP36 available at most FLAPS for about $12-$15. I have LED turnsignal boards on the front and rear of my 930 and the EP36 flashes correctly at about 1 flash per second.

[Tremelune]

Interesting. Would it be possibly to put the resistor on the "flash" circuit only? That way you'd have the benefit of a low-draw always-on light without the hassle of changing the blinker relays. With how rare blinkers are on, I doubt the draw of that circuit matters at all in the scheme of things.

[spoke]

Originally Posted by Tremelune

Interesting. Would it be possibly to put the resistor on the "flash" circuit only? That way you'd have the benefit of a low-draw always-on light without the hassle of changing the blinker relays. With how rare blinkers are on, I doubt the draw of that circuit matters at all in the scheme of things.

Good point about how rare blinkers are on. For a robust design although, one should assume the blinkers are on for an hour or so. Like sitting on the side of a busy highway with a vehicle issue and you want the blinkers on to warn other drivers. That's why the resistors on the market for this application are very large size 50W power resistors.

These load resistors have to be spliced into the existing wire loom somewhere in the L and R wiring. If you can find the common wire at the flasher, then you only need to splice in 2 of these. If you do this at the turnsignal buckets, you'd have to add one at each bucket.

https://www.superbrightleds.com/moreinfo/flashers-load-resistors/led-light-load-resistor-kit-turn-signal-hyper-flash-warning-fix/190/831/

Or replace the flasher. The EP36 is pin compatible. It took me 5 minutes to swap flashers.

Those load resistors are quite a band-aid and even with my experience following a schematic and finding wires, I would not want to add these resistors.

[Arne2]

Originally Posted by TheSt|G

Any chance you could work in some type of "diffuser" for the LEDs? I love the brightness, the plug/play nature, safety, and the lack of a need for a good reflector, but I really don't want the look of "aftermarket" LEDs.


As an example, the right looks massively better to me than the left.

A big plus one from me. I like everything about LEDs, except the LED look. That's all that's holding me back on these.

[spoke]

Optics for LEDs is a big business. I'm not into any of the plastics and optics.

Many surface mount LEDs are nearly worthless without a lens.

Carclo-optics Carclo Optics | manufactures some nice lenses for surface mount LEDs. They can get expensive to the tune of $1-$3 per LED for a lens.

The first LED brake board I did used Cree's XB-D surface mount LEDs. Those LEDs kick ass but they need a lens and are expensive. I put 20 LEDs on a board. At $2 for an LED and $1 for a lens, a pair of boards with 40 LEDs and 40 lenses would have $120 cost before board fab and assembly.

[spoke]

Christmas in August.

911 brake boards are in. We'll start assembling these in batches of 10 sets each. Will post a FS: ad in a week or 2 once we get started.

[Racerbvd]

Nice, I need to sell off some stuff since I need a few sets for my SCs & 914s..

[devinbholmes]

Spoke - I have an extra periscope I can send you to prototype with as long as you need it in exchange for being a tester for a periscope 3rd brake light LED. :-)

And put me down for 2 of everything...one for my '84 and one for my '86 cab.

[spoke]

Originally Posted by devinbholmes

Spoke - I have an extra periscope I can send you to prototype with as long as you need it in exchange for being a tester for a periscope 3rd brake light LED. :-)

And put me down for 2 of everything...one for my '84 and one for my '86 cab.

I purchased a periscope already to test fit. Would love to use the extra one you have. I'll pay shipping if you want to send it to me. I will need alpha testers for these... PM coming.

[devinbholmes]

PM replied to. Periscope is on its way.

[spoke]

Originally Posted by devinbholmes

PM replied to. Periscope is on its way.

Thanks Devin. I did purchase a 3rd brake light for fitment although it will be useful to have a second one just to check.

I went ahead and ordered a couple of PCBs based on the first periscope. Hopefully the one you send me will be the same or similar. I worked out some clips to hold the board in place.

I tried to stuff as many LEDs as possible into the opening of the periscope. The pic below shows the board. The white rectangle on the board is the opening size. This board will have 24 LEDs. Based on the comparison of the brake LED boards (> 3x brighter than bulb), I estimate this board will be > 1.5x brighter than the standard bulb.

Because the light from LEDs is very directional, this board will have a smaller visual footprint than the bulb. The footprint will be the opening of the fixture whereas the bulb radiates light in all direction thus making it's footprint a bit larger. However, the LEDs throw all of their light directly behind the vehicle making them look brighter. This should be ok since the biggest threat to safety is the jackass directly behind you texting/talking/emailing/dazed/sleeping.

The standard bulb burns 21W whereas this board should burn about 2W. I'll have the boards in hand in about 2 weeks. Only 3 boards were ordered so I can check for temperature rise and fitment.

[axvel]

Originally Posted by TheSt|G

Any chance you could work in some type of "diffuser" for the LEDs? I love the brightness, the plug/play nature, safety, and the lack of a need for a good reflector, but I really don't want the look of "aftermarket" LEDs.


As an example, the right looks massively better to me than the left:

+1

I agree the right one looks much better