Building a very bright red LED tail light-
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  1. #1
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    Building a very bright red LED tail light

    I want to build a very simple tail light powered by 8,4V DC.

    I already have the casing which was made for 2 LEDs.
    I don't need flashing or something like this. It should shine directly when it is push button or switch.(I have one at the battery pack)

    Is it recommened to just use the LEDs in series with a resistor?

    Or do you know some driver and really bright red LEDs`?

    I often ride my bike outside the town, but there are no bike lines and atm it is often foggy in germany, so I really worry that some drivers don't see me or see me too late.

    Building the stuff is the part I enjoy the most, I just need some advice of the experts to get a good design.

    Thank you very much

  2. #2
    Reputation: tigris99's Avatar
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    Aug 2012
    First off not having a flasher is a REALLY bad idea. flashing gets attention of drivers far better than just a solid red light. IMO doing one flashing one solid on would be perfect but not so easy to do.

    Without knowing what LEDs or what housing you plan to use its really hard to tell you anything. But unless your just using the cheap and basic 5mm leds (which arent that bright) you CANNOT simply use a resistor, you need a driver.

  3. #3
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    Research, learn, and grow. Agree or disagree with evidence and analysis.

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  4. #4
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    I'm undertaking a project like this at the moment. In fact I have a few experimental iterations on the go. First of the line I'll be using a mobydrv with a set of 3 discrete 3w red leds and driven by a 3-4v supply - probably a single (or 2P) 18650.

    I suggest you don't use a resistor with power LEDs. You can do it but it's too inefficient IMHO. If you are experimenting and don't have a great deal of knowledge about LEDs or maybe matching lenses I would just buy some cheap powerLEDs from a place like DX or LEDDNA.

    Of course if you already have a case you may also have some lenses you may want to match the LEDs to. For a tail light I can't imagine that this matching (to achieve quality of beam) is particularly important. Indeed many who cycle in urban environments feel that there is no need for lenses at all - either using bare LEDs or just some sort of diffuser. Check some of the other tail light threads for examples of these.

    Two power LEDs will make a very, very bright tail light irrespective of which LEDs you choose. Suggest you read the various tail light threads to get a handle on how to configure LEDs etc.


  5. #5
    Reputation: Flamingtaco's Avatar
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    Mar 2012
    Quote Originally Posted by tigris99 View Post
    First off not having a flasher is a REALLY bad idea. flashing gets attention of drivers far better than just a solid red light. IMO doing one flashing one solid on would be perfect but not so easy to do.
    I disagree.

    The biggest issue we have is a driver's estimation of our location and their approach speed to us. To perform this calculation, steady, consistent light sources are needed.

    Flashing lights work against a driver making a good determination of where they are in relation to us and how fast they are approaching us. The only information a driver can glean from flashing lights is 'cyclist or runner ahead'. The change in brightness (on/off/on) of a flashing light, especially bright ones, can also affect a driver's vision. The newer lights that flash but remain on (100%/30%/100%) are better, but still not as good as a solid light with decent lumen output.

    Time to drop age-old assumptions and get with the modern. We are no longer a rare exception, almost never seen by drivers, wholly restricted to riding in our neighborhoods, and never seen after dusk. We need a lighting system that provides visibility similar to a passenger vehicle.

    The tail lamps on vehicles work very well to provide location and approach speed info to following drivers, and we should emulate them. One solid light on the seat post or pannier and one solid light on the helmet give drivers the same visual cues as tail lights.

    Flashing lights can create glare for drivers, resulting in loss of visual detail. As a middle-age driver with average vision, I really hate those things, even when worn by joggers on the sidewalk. They are a distraction, and useless for obtaining useful info. By comparison, even the pathetically weak lights sold in runner shops enable me to determine closing speed due to the runner's motion, unless they've put in in flashing mode.

    Too-bright lights are also an issue. Some lights, when aimed up towards the driver are as bright as brand new automotive LED's with the brake applied. Having lights so bright that the following driver's iris are closing to compensate only serves to reduce their knowledge of what is going on beyond you. Drivers make enough poor decisions passing us, we don't need to help them make more.
    I will suffer no butt-hurt fools!

  6. #6
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    argh...I am writing this for the 3rd time now. I got an error each time and the Back-button also did not work...

    We have some crazy laws in germany, that is why I don't want a flashing taillight. It is not allowed here. Until 2013 all lights powered by batteries were forbidden. Only dynamo. Now the law has changed, but it is even more confusing, because the voltage has to be 6V and the light has to be installed in a way that it can't be taken away without tools (sorry. i don't know the english word) most lights are still illegal, but I don't know anybody who got problems because of this law.

    I have this bike now for 8 month and I rode over 4200km.
    At night I just use the busch& müller front light. It is the modell with 80lux and can be used with 6-42V. The beam is really "cut" at half so that you can be sure not to dazzle the traffic and loose any lights in the sky . My second frontlight is a Yinding (2LED, max 1200...or a little less). I wanted to know how bad it is for other drivers and the beam was pointed on the road but I wanted to see the road as fare as possible without loosing too much light in front of the bike. A friend used this bike, I took another bike ... damn, that was really extremely bright and I decided to use the lamp just off the road.

    The same applies for the taillight. I would test it enough before I use it in traffic.

    I have found some pictures showing the light I would like to use. It is made by "Büchel" and on the casing you can find "6V" and "0,6W" .

    First picture shows quite well where to find the 2 LEDs.
    The red area is a reflector and not transparent. The disadvantage could be that the LEDs must be placed directly on the edge of the casing.
    the second picture shows the light at night from behind.
    Third picture: from the side

    Building a very bright red LED tail light-81qte8dpnol._sl1500_.jpg
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    I will open the casing and take a picture later. so that you can see what is inside.

    What do you think is the best way to make it brighter? (It should still work the way it works now, that means power on= light on, power off= no light. I don't want a extra button or switch I have to press each time I use the light.)

    I found cheap red LED emitter with 1W and 3W. They use 2,2V and a current of 350mA/750mA. Two of them(1W) should be enough? what do you think? If I have now 0,6W that means 0,3W each. at 6V you get 50mA, sound like not very much.

    it should be not to complicated, the power would come from the frontlight and highest voltage is 8,4V. Would be so great if someone knows a driver(or just how i have to connect LEDs and resistor...) and red LEDs that would work with this settings.

    If it should be too bright if you look directly into the light, then you can still drill the lens to diffuse the light or something like this.^^

    greets :-)

  7. #7
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    I did some research and got some solutions. it's nice that I use a DC DC Converter to get the 8,4V I am using at the moment. Input is a 3S battery pack(Bosch, for example for drills) , that means 12,4V max.

    I use 8,4V because of the Yinding front light. But it is no problem to drop or rise this voltage(a little bit) if needed.

    (Bad news: The output voltage will drop with the voltage of the battery pack. That means I need another Converter, this time with Constant current.)

    I did some calculations to the following LEDs:

    8mm LED Red 0,5W (3,4-3,6V, 100mA)
    Two LEDs serial with 15 Ohm Resistor @8,4V I am using atm

    8mm LED Red 0,5W(2V, 150mA)
    Four LEDs serial with 2.7Ohm

    1W LED Red (2V, 300mA)
    Two of them, for example with a LM2596 driver

    wow. 2euro for the converter incl shipping ;-) I will get one

    I think I learned something new today

  8. #8
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    Mar 2011

    I would counsel you against using the dyno taillight housing you've pictured. For one thing, you'd be ruining a perfectly good dyno taillight which I'm assuming you can continue to use and which will provide you with safety and redundancy. Always a good thing!

    The second reason I'd give is that housing uses low-current, low power 5mm LEDs and relies on good optical engineering to make best use of the light generated within. These LEDs require minimal heat sinking and the taillight housing is designed accordingly. You will face unnecessary difficulties in providing adequate cooling within this housing if you intend using 1-3W power LEDs - which I believe you do!

    To achieve what you want all you need a short strip of 20mm x 3mm aluminium. Cut and shape it in a fashion that allows you to neatly mount it just below your dyno taillight (you can use alloy connecting strips hanging down from one or both of the dynolight mounting bolts, hidden behind the dynolight). Wire your 2 LEDs in parallel with appropriate wire leads then connect them to the output of a driver like this.

    Mount the LEDs on your little 20mm strip, using appropriate heat transfer compound or epoxy. Cover the driver with some heavy gauge heat shrink to protect and insulate then cover the entire strip with clear heat shrink (including the driver) and ensure both ends are sealed. Drill a couple of small holes at one or both ends and bolt the end of the hanging mount(s) so it sits firmly & horizontally just below your dynolight.

    Connect the loose wires (securely sealed) to the lead of your battery connector. Stress relieve the wire in some way if you think it might get snagged on something.

    This will give you a super simple, super bright light with each of your red LEDs driven at about 500ma (with the specified driver board). You could find a lower power driver easily enough - this was just the first one I found on ebay. In fact it would be easy and neat (if a bit more expensive) to get a couple of 350ma Micropucks and wire one to each LED and then connect them to your power lead in parallel.

    This will give you a super simple, intensely bright and tough battery tail light. No need for a housing at all. Cost about Au$12-15.


  9. #9
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    Jul 2015
    Thank you for your reply, I will think about this. Don't worry about the taillight. I could buy a new one anytime. It was very cheap and I like the other one(the installed one) better...but heat tranfer, you're right, that's maybe a problem I did not think about. I have changed many light bulbs in the house to LED and they do not even get warm. but my yinding gets warm really fast

    I have already taken the pictures and put them on my they are.

    Building a very bright red LED tail light-bild-2.jpg

    Building a very bright red LED tail light-bild1.jpg

    Building a very bright red LED tail light-driver.jpg

    I don't want to take 3W LEDs.
    They must be much too bright and could even be dangerous for other drivers. Even 1W might be too much.

    My idea was like this.
    I have already bought a chinese DC DC 5A converter(4euro) and it works perfect. The new converter would be also in the saddlebag(with the battery pack), connected to a switch, then to the casing,where I would install one row (behind the clear part)of 5 or 8mm LEDs.
    I maybe have to remove some parts of the casing, but you can't see this afterwards. I think I will buy 3-5 different LEDs(size and power) and have a look which of them suits best. I don't care how much it will cost, if it works good afterwards.

    Edit: Checked the casing again and I think I could not even put 5mm LEDs inside.
    not enough space.

  10. #10
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    Quote Originally Posted by Andreas H View Post
    ....powered by 8,4V DC.........Or do you know some driver and really bright red LEDs`?
    I'm afraid what you wrote originally seemed to suggest that you wanted to use the potential of your 8.4v battery to run what are known as 'power LEDs' (1w or more). Your subsequent explanations however tell us that in fact, you just want to use standard 5mm or maybe 8mm 'T' LEDs.

    If you use the latter (T) and you want to stick to 2 LEDs only then you are highly unlikely to be able to produce a light that is much brighter than the dynamo light you already have!

    The best you could do is to maybe try using what are known as 'High Brightness' LEDs which run at about 100ma (0.5W) rather than the 20ma of the LEDs already in your dyno light. Or maybe try some of the Piranah 20ma SuperFlux LEDs available from the same source.

    You could probably just substitute 2 of either of these for the LEDs already in your dynamo light, run it off the dyno using the original electronics and see if they are brighter.

    If you definitely want to run them off a battery, even if you put your 2 'T' LEDs in series you are still going to be wasting a lot of battery energy and you'll need a Buck driver to soak up the excess power.

    To be honest if you only want to use your standard LEDs I'd dispense with your 8.4v and just get a couple of AA NiMH rechargeables in a box with a switch (from DX) and run 3 of the red 100ma (i.e.; 1/2W) in parallel using a micro puck boost driver.

    Or 1 x red PowerLED with a small heatsink!


  11. #11
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    I start to really like your idea about an aluminium strip with just some holes in it for LED.

    There are fewer problems than by putting them in a casing, this looked easier than it was.

    I'm sorry if all this sounds a bit confusing. A few days ago I started to read about LED, driver...and before this point I really did not know anything about this.
    That means that at an earlier point of this thread I wrote some lines that I wouldn't write today(like the 3W LED thing you were worried about.

    Even now I find it difficult to predict how bright a certain LED will be. At first 1W sounded like not very much, nearly like nothing^^

    I did already order a LED driver, it is a simple DC DC Step down converter CC/DC 3A

    I think I will buy at least 3 types of LED and check out what is bright enough, but not too much.
    I also think for a taillight more LEDs with a smaller power are better then 1 or 2 bright ones.
    0,5W and 1W will be tested and a bigger number of small ones

    You can be sure I'm going to test the safety of the taillight myself by driving behind the bike with a car and to make sure that no one will be in danger.

    thanks for your help

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