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8.17: 20 watt halogen commuting headlight


This article is from the Misc Bicycles FAQs, by various authors.

8.17: 20 watt halogen commuting headlight

From: josh@Happy-Man.com (Joshua_Putnam)

I've also received numerous email requests for details of my
20 watt halogen commuting headlight. I posted this last
winter, so the interest this winter suggests that (a) the
readership has expanded a lot; (b) more people are commuting
by bicycle this winter; (c) the state of the economy makes
people unwilling to pay for a NightSun Team Issue. Let's
hope it's a & b.


Why I Made a Headlight

When I started looking for a good headlight for winter commuting in
the rain and fog of Puget Sound, I saw two classes of light:
grossly underpowered, and grossly overpriced. Determined to find a
middle ground, I looked for sturdy fixtures in local hardware
stores and auto parts stores. After consulting lightbulb catalogs
and checking application notes, I settled on a 20 watt halogen
landscaping light made by Toro. The fixture with one bulb cost
under $20. Its full name is the "Toro Varifocus MiniSpot Light,"
model #52985.

If you need less power, there are many other models of low-voltage
landscaping lights available, from 5 watts up. If you want more
power, you might want two of these 20W lights with one on a switch
for a high/low beam effect. All the landscaping light systems I've
looked at use 12 volts, so batteries are easy to get.

I considered but rejected as too heavy and/or bulky a variety of
automotive fixtures, also in 12 volts. For those with a real
craving for light, motorcycle equipment looks promising.

Power Supply

The light requires 12 volts, meaning it draws 1.67 amps for 20
watts. For my goal of a three hour run time, then, I needed 5.0
amp-hours. To be on the safe side, and because it was on sale in a
mailorder catalog, I decided to use a 6.5 Ah lead-acid gel cell.
This is about the size of a brick, but heavier, and cost $32.
Theoretically, at 1.67 amps it should give me about 3.9 hours
between charges. Runtime declines in cold weather, but
keeping the battery insulated helps.

To charge the battery I used a 12v battery eliminator from an old
radio. With no load this actually puts out a little over 13 volts,
but it can't put out more than 500 mA, which makes it a fairly safe
charger for unattended overnight use. Faster charging is safe if
you buy a good charger, or monitor the battery to prevent
overcharging, but I took the easy way out.


For a switch I got an in-line extension cord rocker switch from the
local hardware store. It's rated 15A @ 130v, so it can easily
handle 1.67A @ 12v. It is not waterproof by any means, but at only
12v there does not seem to be any problem with shorting or

Finally, to wire everything together I used plain 16 ga. zip cord.
For quick-release connections I used 1/4" mono plugs and jacks,
primarily for their great resistance to abuse. To make the
connections somewhat water resistant, I put lengths of
small-diameter bicycle tubes over the inline jacks and plugs, with
about 1/4" of overlap. This setup worked well enough when I
carried the battery on the front rack along with the light, but the
1/4" plugs unplugged too easily on longer cords, so I switched to
polarized automotive quick-disconnects. These are also very abuse
resistant, and also weigh less than metal 1/4" inline connectors.

The battery wiring harness has two legs. One plugs into the light
cord, the other connects to the charger. This lets me leave the
charger connected and just drop it into the pannier next to the
battery. I've considered a modification for touring that would
charge the battery from a generator run through a rectifier and
voltage regulator. A friend suggests a small solar panel charger
mounted atop the rear rack, but I'm not sure how much good it would
do in the Puget Sound area -- direct sunlight is rare much of the

A FUSE IS IMPORTANT: batteries like this can briefly put out
hundreds of amps when shorted, which is enough to set the wiring on
fire, do spot welding, etc. Un-fused wiring is not worth the time
and money it saves. I have an inline fuse holder on the battery's
positive terminal, before the Y where the lighting leg and charging
leg split. I use a 10 amp slow-blow fuse to allow considerable
leeway for user stupidity yet still protect the battery and wiring.


Once I had all the parts together I had to figure out a mounting
system. The light fixture is designed to mount on a plastic stake
in the ground, but the bolt that holds everything together is a
standard carriage bolt. I have tried various mountings for the
light itself. Currently I have a 6" carriage bolt stuck through a
hole in the flat center support of my Blackburn front rack. Above
the rack is a standard nut; below the rack is a broad washer and a
wingnut. Spinning the wingnut off lets me remove the light in
under a minute. To stiffen the long bolt, I have nylon spacers
held tight with another pair of nuts.

This placement of the light does several things. It puts the light
6" above the top of my front rack, so that loading the rack does
not block the light. Also, the higher position gives slightly
better lighting at a distance. Finally, only the bolt obstructs the
rack, rather than the entire light fixture, so there is room for my
ever-present waterproof radio. (The light even doubles as a dial
light so I can zap the commercials on the way to work.)

The battery is in a bag I sewed up out of nylon pack cloth. I made
long flaps on the closure to avoid water intrusion, and put on
velcro hanging straps in three directions so I can hang it just
about anywhere. That way I can move it around if I need to
counterbalance unusual loads on the front or rear of the bike.
Because temperature affects battery performance, and also to avoid
cracking the plastic case of the battery, the bag is lined with
1/4" neoprene foam from a long-dead wetsuit.

For better balance, I keep the battery on the front of my rear
rack. Originally I carried it in front with the headlight, but I
prefer not to have the extra weight over the front axle. Some day
I intend to make a sturdier battery carrier that will mount on
water bottle bosses, getting the weight even lower and more
centered on the bike.

After some trial and error, I decided I like the light set near the
long end of its focal range for average riding, though this varies
depending on the weather, traffic, and road conditions.


A 20W halogen lamp is plenty bright enough for most riding. I light
up roadsigns from more than half a mile away, and can see the road
surface in adequate detail for 25mph riding, even when the ground
is wet. There are still some times when the light can't overcome
the weather completely -- last winter (Dec. '91) I was riding in
fog so dense I could not see the ground beneath my front wheel, and
the light would only show the fog line for 10 feet or so ahead of
me. Fortunately, I had remembered to put on my fog strobe as well
as my Vistalight that morning, so traffic could still see me from
behind. As unnerving as dense fog is at first, I actually
feel safer on my bike than in my car, since road feel and
hearing are much better when cycling and aren't hurt by fog.

I'm still on my first light bulb, so I don't know how long
they last. The spare bulb is very small, about the size of
an ordinary flashlight bulb. As with all halogen bulbs, it
is important to keep the bulb clean, so I carry the spare in
its blister-pack wrapped in a small cotton rag for

Future Modifications & Improvements

Apart from the charging options and battery mounting discussed
above, I've considered a number of changes. First among them is a
tail light. I know I'm very visible with my existing
reflector-mount amber Vistalight, but I may still add a steady red
light. Some drivers have complained to me that just a Vistalight
is somewhat disorienting, since there's nothing for the eye to hold
onto between blinks. This may be an advantage -- I'm not sure.
The unknown may get wider clearance than the known. I'll have to
think about this more.

I may someday add either a simple dropping resistor or some more
complex electronic dimmer to extend the battery life, since many
times I don't need the full 20 watts. On the other hand, every
addition is another source of potential breakage, so maybe I won't.

I'm considering a modified lighting system for use offroad on my
mountain bike. It has no racks, so the light would probably clamp
mount on the stem to take advantage of the front suspension -- I
don't want to replace bulbs all the time.


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