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.
The rest are electronic components, all available from
DigiKey.
* If you're using a 10W or less bulb, 1/2W is ok. Otherwise, 1W is best.
Circuitry
If you plan to build the housing out of halogen lamp tube (35mm
I.D.) I would suggest getting a piece of perfboard, perforated
prototyping board and cutting it into a circle that will fit inside the
tube, then soldering all the parts. This requires a little bit of
foresight, and attention as it's hard to remeber what pin is what.
The opamp is in the center, with the sense resistor and FET pointing
down, the potentiometer has 5 wires because it has a built in switch.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)
Housing & Retainer
Find a thrown-out/broken torchiere halogen lamp, the kind that are
7ft tall and break all the time. This should be trivial, as they are
cheaper to replace than repair. Check that the I.D is 35mm (1.4"). Cut a
piece that is between 3" and 3.5" long that includes a set of threads.
Also take one or two of the interconnect threaded pieces.
One end of the housing should have a full set of threads, the two rings are visible in the background.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.
Final Assembly
To assemble, push the circuit board in, using the heat sink to
hold the board in place (the heat sink presses against the sides of the
housing).
Push the cap onto the end of the housing.
Then thread the inner ring, plug the lamp into the socket (or solder it in).
Then thread on the last ring.
If you don't think you'll open it again, you might want to put a
drop of threadlock or superglue, to keep it from unscrewing. Ducttape
the cap, and hotglue/epoxy the hole where the battery wires emerge.
I soldered on a DC jack onto the end and shrink wrapped it all.
Schematics
Or just click on the image below to get the embiggened BMP
