Rear Light - Design Considerations

[Prolixia]

Hi,

Having spent some time debating between buying a DiNotte tail light (£70, no fuss, and a neat little package) and building my own (probably costing not so much less, all in, and unavoidably an ugly duckling) I've obviously decided to make my own!

My main design considerations are as follows:

1) Bright (naturally!)
2) Wide viewing angle
3) Flashing
4) Lightweight
5) Min battery life of approx 1 hour

My plan is to use a red-orange Luxeon III with a flare lens from L2 Optics. This lens will effectively give me 180 degree horizontal visability, with fairly little leakage up or down. Two questions about these lenses:

1) They sound so ideal for rear lights, I can't believe everyone isn't using them. Have I missed something?

2) It isn't clear from the datasheet whether they can be used with a star, or just with an emitter. They're described as suitable for use with "all emitters" and mount driectly to the PCB, so I assume I can use them with a star. Does anyone know for sure?

I'm planning to drive it using rechargeable batteries 4 AAs, or a pair of CR123s. To keep things as light as possible, I'd like to go the rechargeable CR123 route, The light is just for commuting and recharging every day at work is no great hassle. Will CR123s meet my needs? Is it a choice I'll kick myself for later on?

In a slight departure, I was considering using a regular lens (i.e. not a flare) but covering it with some kind of 3D diffuser. If you can imagine cutting a pingpong ball (or something less meltable) in half and putting it over the lens, that's pretty much it. Is this crazy talk?

The rationale behind the ping pong ball idea is that I don't actually need the light to project at all. All I really want is a very bright spot on the bike that is visable from any angle. I imagine the ball being fairly uniformly bright from any angle, whereas a lens would dazzle people directly behind me and underwhelm anyone looking at it off-centre.

Finally, what about drivers? 1A drivers are easily had, but looking at the Luxeon III datasheet (page 11, second graph down) it looks like I'll be sacrificing 23% of the red-orange star's brighness for driving it under the full 1.4A. Having said that, I've had no luck finding a 1.4A driver. Alternatively, is it worth forgetting about the driver and just driving it directly (especially since I can recharge every day)? Using a driver does have the advantage that I can use a built-in function for flashing the LED.

Any comments/thoughts are very welcome before I start buying parts. I'd also appreciate recommendations for a suitable driver (especially one easily obtainable from the UK!)

Many thanks,

P

 

[FrontRanger]

Finally, what about drivers? 1A drivers are easily had, but looking at the Luxeon III datasheet (page 11, second graph down) it looks like I'll be sacrificing 23% of the red-orange star's brighness for driving it under the full 1.4A. Having said that, I've had no luck finding a 1.4A driver. Alternatively, is it worth forgetting about the driver and just driving it directly (especially since I can recharge every day)? Using a driver does have the advantage that I can use a built-in function for flashing the LED.

Any comments/thoughts are very welcome before I start buying parts. I'd also appreciate recommendations for a suitable driver (especially one easily obtainable from the UK!)

Hi Prolixia,

It's a matter of opinion, but I don't think you really need the full 1.4 A. The red Lux III (LXHL-PD09) has a nominal Vf of 2.95 V @ 1.4 A. That's about 4W of input power in a taillight that's used for being seen, but that's more than many headlamps that are used for seeing. (Note the rare event here: It's not normal on CPF for someone to say "you don't need that much light.") And it might be nice to have the full 1.4 A, because it is not being focused. But I think there's a good chance you'll be quite visible with 700 mA or maybe even 350 mA. Drivers with both of those current values are readily available. Check out the offerings from Luxdrive (BuckPuck or BoostPuck, depending on your battery choice), the various drivers from Taskled, or even the cheaper models at dealextreme.com and kaidomain.com. I know you want to only order parts once, and I'm not sure about shipping to the UK, but you may find that some of these are cheap enough to be worth experimenting with. If you can get away with less power dissipation, why not go with it?

And if not, you can always shunt multiple current-mode drivers together. Two 700 mA drivers with the outputs in parallel will give you the 1.4 A.

Good luck, and :welcome:

 

[Prolixia]

Thanks for the feedback.

As an alternative to trying to drive a single Lux III with 1.4A, would I be better off running two in parallel from, say a 700mA or 1A driver? I could see that giving me more lumens and less heat - plus I could fit one LED with the 180 degree flare optic and one with a regular wide angle lens (since *most* of the traffic will be approaching from directly behind).

Does anyone have any thoughts re. using some kind of 3D diffuser? I was thinking of taking a clear plastic hemisphere and spraying the inside with non-etching glass froster.

Cheers,

P

 

[FrontRanger]

2) It isn't clear from the datasheet whether they can be used with a star, or just with an emitter. They're described as suitable for use with "all emitters" and mount driectly to the PCB, so I assume I can use them with a star. Does anyone know for sure?

I wanted to know the answer myself so I emailed luxeonstar.com. The answer is that they can NOT be used with a star. Bare emitter only.

 

[FrontRanger]

As an alternative to trying to drive a single Lux III with 1.4A, would I be better off running two in parallel from, say a 700mA or 1A driver? I could see that giving me more lumens and less heat

Running at those lower currents will indeed give you more lumens per Watt. Don't expect a dramatic improvement, but there will be one. You can probably find the data somewhere for luminous flux and voltage as a function of current and calculate it exactly. Also, it will be easier to get the heat out since it's more surface area per Watt.

plus I could fit one LED with the 180 degree flare optic and one with a regular wide angle lens (since *most* of the traffic will be approaching from directly behind).

One possible problem there is mounting. If they were side-by-side, you'd lose out on the flare somewhat. Maybe wide angle on the seatpost, flare on the rear rack?

Does anyone have any thoughts re. using some kind of 3D diffuser?

Experiment. Dealextreme and kaidomain are cheap.

 

[dnlwthrn]

I'm planning a taillight very similar. I ordered the LuxIII red-orange yesterday, but went with the Carclo elliptical lenses. I'm thinking of doing a frost on them somehow to diffuse them slightly, and will run 2 or 3 (not sure yet, have 3 coming) to give more effective light area. I think that's the biggest thing with a taillight; you need more surface area "lit" to make you more visible along with the brightness to catch motorists' eyes. I won't have a blinkie option at this point, as I'm planning on using a Fatman driver that I already have in my posession. I might consider getting a maxflex or similar down the road, but for now it'll be a 2 level setup.

 

[Prolixia]

The answer is that [Flare optics] can NOT be used with a star. Bare emitter only.

What a pain - that's not very clear in the datasheet. I'm reluctant to use an emitter because of the additional considerations of choosing a suitable PCB etc. (i.e. how to couple the emitter to a heat sink). I wonder if I could just cut down the board on a star...

Running at those lower currents will indeed give you more lumens per Watt. Don't expect a dramatic improvement, but there will be one. You can probably find the data somewhere for luminous flux and voltage as a function of current and calculate it exactly. Also, it will be easier to get the heat out since it's more surface area per Watt.

That sounds good. The real advantage for me, though, is increasing the total brightness without needing a higher current source. I'm thinking a single 1A/700mA driver with two Lux IIIs running off it will be easier than trying to hack together a 1.4A source (e.g. with more than one driver).

As for mounting, I'd planned to put both LEDs on the seatpost, but one above the other (I only need the horizontal to be clear for the flare). I think that's the most compact way to do it.

Thanks again - I would have ordered a flare with a star had it not been for your timely comments!

 

[Prolixia]

I'm planning a taillight very similar. I ordered the LuxIII red-orange yesterday, but went with the Carclo elliptical lenses. I'm thinking of doing a frost on them somehow to diffuse them slightly, and will run 2 or 3 (not sure yet, have 3 coming) to give more effective light area. I think that's the biggest thing with a taillight; you need more surface area "lit" to make you more visible along with the brightness to catch motorists' eyes. I won't have a blinkie option at this point, as I'm planning on using a Fatman driver that I already have in my posession. I might consider getting a maxflex or similar down the road, but for now it'll be a 2 level setup.

Sounds cool! I think you're probably right about the visible surface area.

Let me know how you get on with the frosting. I'd be interested to know whether you get a beneficial effect from frosting the lens itsslef, or whether you find it better to frost a diffuser mounted in front of it - to capitalise on your surface area idea. I'm thinking something like this would be good for all-round visability, but perhaps more mushroom-shaped (for a large visable area from the rear, but still something from the side).

P

 

[znomit]

If you can imagine cutting a pingpong ball (or something less meltable) in half and putting it over the lens, that's pretty much it. Is this crazy talk?

I think thats a bloody clever idea. Excellent viz from all around. :twothumbs
I'd do one steady led like that and another flashing with a 6x25 optic.
Personally I wouldn't run at high powers. 200mA should be plenty.

 

[Cemoi]

I (...) went with the Carclo elliptical lenses

Where did you buy these?
The Carclo online store only sells large quantities.

 

[dnlwthrn]

Cutter, so it will be a bit before I get the stuff (my average is around 14 days from them).

My latest idea is to angle the base slightly (maybe 5 deg?) for each light (imagine it looking like this from the top /-\ but only a 5 deg angle) and then set them back in the housing a bit and use sections of automotive taillight housing to increase the surface area. I'm thinking I could find something at the local wrecker for cheap and then cut what I need out with a dremel. If you've seen the "amoeba" design over on MTBR you'll get a basic idea of the whole housing...

 

[sping]

2) It isn't clear from the datasheet whether they can be used with a star, or just with an emitter. They're described as suitable for use with "all emitters" and mount driectly to the PCB, so I assume I can use them with a star. Does anyone know for sure?

I'm not so sure they're a dead loss... I have the star and flare lens, I just haven't put them together into a light. There's no reason it can't sit on top of the star - I was planning on epoxying it on. The only thing is it doesn't have a huge surface area to sit on.

I'm going with a dynamo, so I don't need any electronics (aside from the rectifier, elsewhere). I'm planning to find some 1 inch (internal) U-channel, stick the star to it, and the flare on top, glue/silicone the gaps. The lens is fractionally less than 1 inch high, so if I find the U-channel, it'll be a nice fit. The sides of the U-channel will go top & bottom of the flare. I'll cut/file it to the semi-circular shape.

I think with the flare lens you may want the full power. Sure, that's a lot of light, but it's spread over a very wide area. I'm only likely to get 500mA from a dynamo, and my rough estimates are that it will appear significantly less bright than a PB Superflash in any given direction. I think on max power it might approach that brightness, which is what I'd like to have. I am going to see what it's like, and I may make a double unit.

I'm planning on doing it fairly soon, and getting a dyno hub (Sturmey Archer, with Drum brake). In the meantime I might try to get it working with some $10 bottle dynamo. Newborn and toddler prevent me getting to this job...

Edit: oh yeah, and I'm thinking of putting an MC-E behind a flare lens in front, for another be-seen light! Hub dynamo could drive the 5-6 emitters nicely! While an MC-E behind a 10 degree optic would be obnoxious on the road, in any direction behind a flare it's going to shine less than 1/10 of that power.

 

[FrontRanger]

What a pain - that's not very clear in the datasheet. I'm reluctant to use an emitter because of the additional considerations of choosing a suitable PCB etc. (i.e. how to couple the emitter to a heat sink). I wonder if I could just cut down the board on a star...

I think the problem is as sping says downthread: You could try to mount the lens to the star MCPCB, but not much area. The intended mount for the lens is through those two holes. By the way, you needn't mount the LED to a PCB, just thermal epoxy to appropriate metal (usually aluminum).

Thanks again - I would have ordered a flare with a star had it not been for your timely comments!

Sure. I've received plenty of help from CPF-ers myself.

 

[1 what]

Hi guys,
Been watching this develop over the last few days.
Have a look at posts 67 and 69 in:

http://www.candlepowerforums.com/vb/showthread.php?t=147209&page=3

I'm still using it and still on my first set of 123's.
It gets about 5 hours use perweek and I'll replace the batteries at Xmas just for the heck of it as I reckon a years use is good value and I don't want them to go flat on me.

While I'm at it I'm going to remount the rod vertically and taper the ends at 45 degrees to max the all round visibility.

 

[Prolixia]

Hi guys,
Been watching this develop over the last few days.
Have a look at posts 67 and 69 in:

http://www.candlepowerforums.com/vb/showthread.php?t=147209&page=3

I'm still using it and still on my first set of 123's.
It gets about 5 hours use perweek and I'll replace the batteries at Xmas just for the heck of it as I reckon a years use is good value and I don't want them to go flat on me.

While I'm at it I'm going to remount the rod vertically and taper the ends at 45 degrees to max the all round visibility.

That's really interesting. I love the way the rod acts both as a lens to people immediately behind and provides good side-on visability.

 

[dnlwthrn]

Looks pretty good. I wonder if I can find anything like that laying around here... I've already ordered a couple of the elliptical optics for my setup, but if I can find some spare rod laying around I'll test things out with LuxIIIs.

 

[Offroad'Bent]

I'm still disappointed nobody has a simple red flashing 3W Cree 2AA flashlight like a Fenix for a self-contained daylight-visible rear light.

 

[mkrabach]

I have some information that you might be able to use considering the options for a good rear light. You may have found them elsewhere on this site but here is the link. You want to look at the later Prototype lights 10 and on. The lights out did any commercial I have seen lately or on the candle power forum. I cant understand why commercial mfg have not done similar lights. Maybe just too bright?
http://www.kayakaccessri.info/krabachwebsite/bike_light/bike_light_project/intro.html

 

[Offroad'Bent]

Re: Rear Light - Design Considerations- Zebralight possibility.

I just heard from Zebralight that they will be offering a flashing Cree single AA light.
"We will provide flashing red versions of our current products. Please check back
our website later for announcements.


Thanks and regards,

Lillian Xu
Customer Services
ZebraLight "

 

[Prolixia]

@mkrabach

Cheers - I've in fact seen your site whilst I was researching lights, but I've taken a closer look now.

I'm impressed by the number of differnet protoypes you've built!

P