Introduction: Kilo-Lumen Bike Headlight

About: I'm an electrical engineer. By day I design chips, by night I like making stuff that is unnecessarily complex.

I started biking to work this summer and needed a good headlight and taillight. I didn't want to spend a lot, but I wanted extreme visibility. For about $150 I ended up with a headlight that puts out somewhere around 1200 lumens, and a really effective tail light. The power source is an 18 volt Ryobi power tool battery which is both easily replacable, and quickly charged.

Warning! This project is not a beginner's project. As such I'm not documenting it as such. This will be more of a guide and list of critical elements rather than a comprehensive How-To.

Step 1: Design Choices, Thermal Considerations

The first step is to determine what you want/need for your bike light. Total light output and beam shape will determine the type, number, and configuration of LEDs needed. Additionally mounting, portability, weather resistance, and access to machine tools will weigh in.

I started out with the intention to build the brightest lights around for the purpose of being noticed. This means for the headlight over 1000 lumens (although there are a few 1000+ lumen bike headlights commercially available, they cost close to that amount in dollars).

I settled on using 6 Cree XR-E LEDs, which from the appropriate bin will put out in the range of 180 - 230 lumens each at a 1 amp drive level. (UPDATE: The new R2 bin XR-E LEDs put out up to 275 lumens) This gives me a headlight which (ignoring losses from the lenses) will put out between 1080 and 1380 lumens. A number of considerations have to be made when using an array of LEDs of this magnitude.

- At 1 amp, these LEDs will requrie about 22 watts of DC power, carefully regulated to avoid over current and overvoltage conditions.

- With the LEDs running somewhere under 50% efficiency, the array will dissipate somewhere between 10 and 15 watts of heat. This must be disposed of properly to keep the LEDs within their rated junction temperature limits.

- Having 6 emitters will allow customization of the beam pattern. Each LED will have its own lens. The end result is a superposition of narrow spots, medium spots, and wide angle oval. This ensures side visibility, while providing good, even lighting towards the path ahead.

- A suitable power source needs to be provided. The array is a series connected string of 6 LEDs, each with a Vf in the 3.7v range, meaning that a power source in the range of 15 to 25 volts is required (This is to keep the regulator from working too hard to boost or buck the native supply voltage.)

- With the weight of the 6 LEDs, a suitable heat spreader, and heat sinks the headlight is going to have a pretty decent mass and needs a solid, adjustable mount that allows for quick dismount when parking the bike outdoors.

- Brightness control is handy so that when you are riding towards oncoming cars you don't piss off drivers by blinding them. This light is so bright that in darkness it can be blindingly bright. A measure of caution and restraint is needed when using a light this bright.

Step 2: Bill of Materials

6 Cree XR-E (Q5-WC bin) LEDs mounted on carrier PCBs
<<<< Update - Technology marches on. I built this light back in August and since then Cree has come out with a new higher bin XR-E, the "R2" These put out up to 275 lumens versus 240 for the ones I'm using. Alternatively you can use the new MC-E which i 4 dies in a single optics package. Each one of these puts out ~700 lumens. A wide selection of optics exist now for that as well, and it is easier to drive than the SSC-P7 (another 4-die part). >>>>

1 BlueShark current driver
1 10 kohm potentiometer with built-in switch
1 barrel type connetor pair
1 hookup wire (I used 20 guage, 22 should suffice)

1 package of Artic Silver Epoxy (not the grease, but the two part silver loaded epoxy
1 3 inch long section of 2" by 3" by 1/8" thick rectangular aluminum tubing
1 3" by 2" by 1" block of aluminum
1 3" by 2" by 1/4" plate of aluminum
1 3" by 2" by 50 mil sheet of aluminum
12 4-40 flat head 1/4" screws
1 bike mount
6 Lenses of your choice

For the Battery
1 Ryobi 18 volt battery
1 Ryboi 18 volt flashlight
1 Amphenol Power PowerPole connector pair
Silicone sealant
JB weld

For the taillight:
18 Lumileds Pirhana LEDs
1 Taskled CC1W current driver
1 scrap housing
1 piece of lexan or other suitable clear plastic
1 Silicone adhesive/sealent
a bunch of screws to hold it together


LEDs and lenses:

BlueShark Driver:

Taillight Driver:

Handlebar mount:

I bought the alumnium from a local metal supply place. I take a trip every month or so looking for interesting remnants and have a decent stockpile of pieces now. I designed the light to fit the materials I have. There's nothing special about the dimensions except that they lenses fit well.

Step 3: Basic Diagram of Headlight

Before getting started, here's a basic diagram of the headlight, mecahnically and electrically. I'll add more detail in subsequent sections, but for now this gives you an idea of what things do and where they go.

Step 4: Machining the Heat Spreader

The heat spreader is a crucial component of this project. The heat spreader must conduct 10-15 watts of heat to the outer housing which acts as a heat sink. This means that the dimensions of the spreader are critical. Gaps between the spreader and the housing will result in poor heat conduction resulting in a potential overtemperature condition within the LEDs.

I used a Harbor Freight table-top milling machine to shape the heat spreader. It didn't come out perfectly but it came out close enough and with the use of screws on the side of the housing, it works fine.

The front face of the spreader is just as flat as possible and has holes for power supply wires. Additionally there are holes tapped for 2-56 scrwes used to hold the LEDs in place while the epoxy cures.

I overdid it with the silver epoxy, evidenced by the look of a flooded surface. The compression of the screws ought to maintain a reasonably thin interface between the LED PCB and the spreader. I reccomend using an ink roller and rubber ink pad to spread a 1-2 mil layer of silver epoxy.

The back of the heat spreader is machined out to make room for the driver circuit. The first driver I used required a small pillar for heat sinking. I blew this driver up, and changed to another type (the BlueShark) which comes with a copper heat spreader. I reccomend this driver over the Taskled maxFlex (which I burned out) simply because the Blueshark uses a potentiometer for brightness control. The maxFlex is a really great board but I like the variable control.

Alignment of the LEDs is critical. The square lenses I used are almost 1 inch square. This means there is little room for error in the alignment of the LEDs. Careful layout with a set of calipers should provide adequate alignment. This is another useful feature of the scrwes, as they keep the LEDs in alignment while the epoxy cures.

This heat spreader goes into the blue housing and is secured on the sides by screws. The sides provide the majority of the heat conduction and as such, the wider the cross section the better (hence the 1 inch thickness of the spreader).

Step 5: Machining the Housing

This is yet another step where I assume some knowledge of milling machine techniques.

I used a 1/8 inch end mill to machine 70 mil deep channels across the length of the housing. This provides heat sinking and more than doubles the thermally emissive surface area of the housing. The channels were cut front to back with respect to direction of travel of the bike. This allows the airflow from riding to dissipate the heat better. Cross cutting in the other direction resulting in a checker board pattern would increase turbulence and potentially increase cooling effectiveness. I have found though that after my 15 minute commute on cool evenings that the housing is just barely warm to the touch.

There are some excellent comments below regarding the coating of the housing. It turn out that for an application where convective airflow is the dominant cooling mechanism (as is the case here) that coating the surface of the aluminum will not measureably improve its heatsinking performance. In any case I decided to powder coat mine for durability and aesthetics. As stated above, the cooling performance of the housing is sufficient, so overall it seems to be working fine.

Step 6: Machining the Slide Mount

The slide mount that I use is from PlanetBike. They sell it as a replacement for a few dollers. The great thing about their site is that they encourage the fixing of old or broken headlights rather than replacement. Thus they offer complete replacement parts for their products. If only more copmanies would take that approach!

Anyway, lest you think I am getting paid... I had bought a really crappy Planet Bike headlight a few years ago and decided to use the same old mount which is actually very well made.

I just needed a matching base for my headlight. I took measurements from the headlight that I had and replicated it in aluminum.

Here's where I will update this with a diagrammed drawing at some point, but to summarize, the slide is about 100 mils thick, and just under 0.75 inches wide. Make sure to machine a small gap for the locking hook.

I made this slide from two pieces. 1 piece is that 1/4 inch aluminum plate called out in the BOM. The other is the 62.5 mil sheet called out. Screws hold the two together as seen in the picture. Again, the milling machine came in handy here, this time with smaller end mills (1/8 inch I believe).

Step 7: Control Panel, Wiring, Etc.

The control panel is made from another piece of that 62.5 mil sheet, and has holes in it for a power LED, a brightness control pot, and a cheapo barrel type DC input connector. This is held onto the heat spreader with standoffs and the whole thing slides into the housing and is held in place on the sides by the 8 4-40 screws.

Wiring of this is simple. If you've gotten to this step you will have the regulator board which comes with hookup instructions. To use an external pot, there are also links to threads on CandlePower Forums about that...

As a word of caution, even to the most experienced builders, CHECK YOUR POLARITY and then double check it. This is how I blew out my first driver board. Don't be a dummy like me!

Step 8: Lenses

So the lenses I'm using here are really cool because they come with their own mounting solution. I discovered these lenses (and derived inspiration for this project) from the excellent web site . In fact, I should have mentionted the site before because the design and construction techniques featured there are much simpler and require no milling machine.

The lenses are made by Ledil and available through Order a variety of lenses and figure out what works for you. I chose a combination of spot sizes (narrow, diffused medium spot, and wide oval).

To mount the lenses, peel off the white backing to reveal an excellent adhesive strip. Some work is required to make room for the solder connections to the LEDs, but once thats done, stick in place and hold for a moment.

Step 9: Fire It Up!

After finishing my headlight I wanted to see just how bright it was. The following pictures show the headlight (center) and the high beams of an Audi TT (with HID headlights). Essentially the Kilo-Lumen headlight is as bright as the car's highbeam, maybe a little brighter, and certainly with a higher color temperature (the high beams on the car are halogens).

Step 10: Battery

Powering a headlight like this requires a substantial amount of power, enough that if you were to add it as resistance to pedalling by means of a generator, it would be very noticable. I chose to use what I had, namely power tool batteries. They are cheap, available everywhere, and in my case you can get LiPo replacements for much higher capacity.

The challenge with using power tool batteries is the connections. The freebie that comes with many of these power tool kits is a flashlight and I found that I never used it. So with one quick slice with a hacksaw (well many actually) I transformed the flashlight into a battery adaptor. I added a powerpole connector to the adaptor and epoxied it in place. I also sewed a pouch to carry the battery on the frame of the bike. A small aluminum cap glued to the top provided debris protetion until it fell off. I haev yet to bother to replace it.

Remember to fuse your conenctions. I didn't but then I'm just lazy. Or maybe I'm hoping that someday I can increase my visibilty riding to work in one bright flash of flame.

Step 11: Taillight

Possibly the most important light on a bike, most taillights are woefully inadequate. 18 Superflux/Pirhana LEDs work well, and if you have a rack on the back of your bike, its a convenient place to mount it.

This build is really easy. Find a surplus metal housing, gut it, cut out a plastic cover, and glue in the strips of LEDs. Of course first you have to find strips of LEDs, or make a PCB etc. In any case, its a box with LEDs, a Taskled CC1W current driver and thats it. People haev told me I look like I'm on fire. Its as bright as the LED brake light of many cars, and its wide dimension distinguishes itself from the majority of bike taillights. I also have this running off the 18 volt Ryobi battery.

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