Dimmable Li-Ion Halogen Bike Light

Biking in the winter, or at night, is dangerous without a front light. A good front-light is essential, not only for being seen, but to see the road. LED lamps will not illuminate the road, and they can be too dim for cars to see you (1W or better LEDs are quite nice as headlamps.) I offer here a simple (but high-quality) design to build your own 5 or 10W halogen lamp, which runs off of a rechargeable 7.2V or 7.4V Lithium Ion battery pack.

Assembled light, with 10W halogen, and 3.5Ah 7.2V lithium ion battery pack (the nice battery holder was custom made for another project).

LiIon vs. Other battery types

LiIon are extremely light and small, much lighter/smaller than NiCads, NiMH and for sure Lead Acids. They can handle a couple hundred recharge cycles, are readily available (unlike LiPolymer) and work in cold weather. Another nice thing is their voltage starts at 8V and quickly drops to 7.2V, then stays there for most of the life, before dropping slowly to 6V. This means that overvolting your 6V lamp is really easy. The bad thing is that you may not notice that the lamp is dimming before it abruptly turns off due to the internal undervoltage fuse.

Li-Ions are a little more delicate than most batteries and must not be over or undervoltaged. I use prepackaged batteries with protection circuitry inside. I would hesitate to use bare-back cells, as fires suck. Regardless, Li-Ions dislike giving burst charges, so you can't just connect your lamp to a battery with a resistor (or potentiometer) in series, as halogens will current-spike (5A? 10A?) if voltage controlled. Therefore you must use a current-control scheme as presented here.

You can buy Sony InfoLithium/Camcorder batteries (such as the NP-F550/F750/F950 formfactor) off of EBay for cheap, you can connect to them with mini banana plugs. (Or by soldering directly if you are really really quick and have everything pre-tinned, I don't advocate it unless you're safe!)

Halogens vs. Other lamp types

There isn't really any other reasonably priced options available right now, 5W luxeons are $50 and the white ones don't last more than a couple hundred hours. They are more efficient than the MR-11's I use, but the best is really to go with an MR-16 10W if possible. I had trouble finding a 6V MR-16 bulb, so I stuck with MR-11. Most 'professional' bike systems use MR-11's because they are small and unobtrusive.

You can overvolt by turning up the pot, for up to 10% overvolting (5% over-current), which will give you 40% more lumens but give you only 1/3 the bulb life. For more information about overvolting see page 8 of the GE Halogen Guide.

These bulbs are available from Atlanta Light Bulb for $6 each, make sure to get the right voltage, wattage, and with a coverglass.

* If you're using a 10W or less bulb, 1/2W is ok. Otherwise, 1W is best.

Download the schematic from the download page. Look at it and place all the pieces before soldering anything! Place the 8-DIP opamp first, about center, then place the FET, on the perimeter so that the heat sink will press against the housing, then place all the resistors. Remember that the 0.1 ohm resistor must be precise in value and 1/2W or 1W (at least) otherwise known as a 'sense resistor.' In the pictures, the white ceramic tablet is the resistor, and the metal sheet is the heatsink for the FET.

Preferably, the potentiometer and on/off switch should be in the same package. Make sure the pot is linear taper, not log/audio taper.

Use solid-core wire when making jumper-wires on the board, and stranded for wires that go to the pot/switch/lamp.

The other side of the circuit board, showing the heat sink, and the large sense resistor. Also, the lamp socket is visible in the top lefthand corner.

To connect to the lamp, you can either (carefully) solder directly, or get a socket. The lamp should last a really really long time (1,000hrs at least) so there's nothing particularly wrong with connecting directly to the lamp. Just make sure you can still assemble it (read the housing assembly instructions since a ring needs to go between the board and lamp).

I didn't include a fuse, but you may want to. (A proper LiIon battery is current limited anyways, which is why the whole system is current-controlled.)

Use heat-shrink liberally when necessary, not electrical tape. Electrical tape doesn't hold as well.

Plug in the lamp and give power to the system, verify that it works, there should be a ~150mV drop across the resistor, and a voltage that is 32 times that on pin 7 of the op amp. There should be 5V at pin 2. Turning the pot should dim/brighten the lamp. (Hopefully)

Cut the interconnect so that you have two rings with three threads (or one with four and one with two). Debur/file everything. Thread one of the rings in all the way in, then place the lamp in so that the ring holds it and thread the other piece in, verify that they are the correct size.

One of the filed rings inside the housing.

You can put a gasket in if you'd like, to make it more waterproof.

To cap the other end, find a plastic jug cap that is 1.5" diameter. These are surprisingly common, and I got mine off of a thrown out motor oil container. Cut the threads out with a dremel or knife. Drill a hole and mount the potentiometer. Drill another hole to let the two battery connect wires through.

I soldered on a DC jack onto the end and shrink wrapped it all.

Schematics

• As an Eagle schematic file