Davolite: A lightweight 3W LED headlamp for nighttime cycling

October 13, 2006

I've been playing around with Luxeon LEDs for over half a year. My first project with them was creating a lighting system for my 10-speed bicycle (I'm working on a webpage for that....slowly). More recently I've thrown together a small and lightweight, but extremely bright light for nighttime cycling (or other activities). At the 2006 Massbike Festival in August there was a tent there where Dan Surkis was showing off and selling his Danolite helmet light. It was an amazingly small light, both in size and weight, which impressed me. I immediately recognized that it was just a high powered LED like the Luxeons I've used and a battery pack. The new multi-watt LEDs that have been introduced in the last few years are incredibly cool and I love seeing products using them. I decided I could make my own for considerably less money than the Danolite was selling for ($99.99, which also includes batteries and recharger). I don't know the internals of his Danolite, but from what I could gleam from the Danolite website and speaking briefly with Dan, I suspect the LED is driven directly from the batteries, perhaps with a small resistor to help set the current. If anything, there would probably only be a 1 to 2 Ohm resistor in series with the battery. That keeps the electronics extremely simple.

Parts List

  • Luxeon III Star 3W LED from www.lumileds.com (part of Future Electronics)(part# LXHL-LW3C): ~$7.00
  • 6' 3.5mm M-F extension cord from All Electronics(part 3CB-333): $1.60
  • 4xAA battery holder with cover and switch from www.jameco.com (part #216186): $1.09
  • Lens and holders for Luxeon LEDs from www.ledsupply.com: One lens and holder: $2.80 Luxeon lens & holder PDF
  • Four rechargable AA batteries (with greatest mAh you can find to increase run time)
  • Velcro. One .75-1" wide and about 15" long strip of the dual purpose (has both the hooks and loops on the same side, so it can stick to itself) velcro. One 3-4" long .75-1" wide strip of dual purpose velcro. Glue or sew the small piece to the back of one end of the long piece. This is the strap used to secure the light to the helmet. Also need one small 1" square of velcro that will be used to secure the light to the velcro strap.
  • Aluminium angle- I bought a 4' piece of 1" aluminium angle at a local hardware store. It can be relatively thin because aluminium is light, but strong. I cut off a 1" piece of the angle and on one outside I glued the LED assembly and on the other outside I glued a 1" square piece of velcro.
  • Strong Glue. I used Elmer's Polyurethane glue (same stuff as Gorilla glue, but quite a bit cheaper).

    Tools Needed

  • Soldering iron. This is to solder the cable to the Luxeon LED and on the battery end to desolder the wires that the battery case comes with and to solder on the cable wires instead. You'll also need some solder too.
  • Wire stripper. This is to strip the end of the cable wires for soldering. A sharp knife can work too if you're careful.
  • Small clamps. These are to hold the pieces together as the glue dries.
  • Small philips screwdriver. The battery case has some small philips screws that secure the cover and internals.
  • Drill. To drill a small hole (for the cable) thru the LED holder and also to slightly widen the wire hole (for the cable) into the battery holder.

    Making the light

    I'll try to write up something later on this, but, for right now, look at the pictures and read the parts & tool info.
    Electrical design- Using Ohm's law (Voltage = Current x Resistance) we can figure out what value resistor we should use to set the current for the LED.
    Vbat = I x R + Vled
    Vbat = 4xAA = 4x1.2v = 4.8v
    Vled = ~3.6v
    Want current (I) to be around 830mA (based on 3.6Vled, .83A * 3.6v = 3W).
    Solving for R, we get:
    4.8 = (.830 x R) + 3.6
    1.2 = .830R
    R = 1.44 Ohms
    I used a 1.2 Ohm 1/2 watt resistor in my design and the length of cable should also provide some resistance. Really the resistor should have a higher power rating, but hopefully it'll work allright.
    So the LED is hooked up almost directly to the battery, with only the small 1.2Ohm resistor in series. The Luxeon LED anode (+) and cathode (-) are clearly marked, so is simply a matter of wiring the battery pack anode (+, red wire) to the LED anode (+) and the battery cathode (-, black wire) to the LED cathode (-). In the battery case red and black wires were removed and the two shielded conductors in the cable were soldered to the battery case terminals (actually a 1.2Ohm resistor was inserted in series between the battery case switch output terminal and the positive cable wire, see the pictures below). On the LED side, the two shielded conductors of the cable were soldered directly to the LED. The 6' extension cable was cut approximately 15-18" from one end. This short end is connected to the light. The longer end is connected to the battery case. Then the two original ends of the cable can be connected to complete the circuit. The cable is a stereo cable, so it actually has three conductors in it, two shielded wires and an unshielded wire for ground (which we don't use in this project. One thing to keep in mind with the aluminum heatsink on the Luxeon star emitters is that it is not electrically neutral, but that shouldn't cause any problems in this project.

    These are pictures of the Luxeon holder and a collimating lens. You can buy lenses with different beamwidths. I used a 15 degree collimating lens.

    This is what a Luxeon emitter looks like. This is the star package (about the diameter of a quarter) and comes on a PCB with a metal base for a heatsink (these high power LEDs can generate some heat, so make sure they have proper cooling)

    This is looking down into the Luxeon holder that has been glued to the Luxeon and also to the aluminium angle. The lens is removed and you can see how the cable is soldered directly to the Luxeon LED terminals. I used white wire for positive and red for negative.

    These next two photos show the light assembly, with the lens installed (it just snaps in).


    The next two show the inside of the battery case. Because the case also includes an on-off switch, there is a little extra section at the top of the case to accomodate the switch. To access this section, I had to remove a screw and pry apart some fused plastic parts, but it wasn't that hard. The first photo shows the section with it's cover removed. You can clearly see the 1.2 Ohm resistor I added in series. The left side of the resistor connects to the switch. The right side to the cable.The second photo is the same picture, but with the section cover installed.


    This is the front side of the battery case where the on-off switch is located.

    Two photos of the battery case and the light. In the second photo, the black strip in the middle is the velcro mount for the helmet.


    Couple shots of the light mounted on my bike helmet.


    Here's what the light looks like attached to a bike helmet.

    Here are some photos comparing the davolite with some other flashlights. I'm no camera expert, but I think they should pretty accurately reflect their light output. I set my digital camera to a shutter speed of 1/4 for these.

    LL Bean Wind'n'Go Flashlight (3 white LEDs)

    3D cell Maglight (normal halogen bulb)

    Davolite

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