My Cheap 20 watt Bike Light
by Michael Krabach

[Additional Variations and Safety Suggestions]
Updated 2-10-2008

[The LED Bike Light Experiments]
Created 2-12-2008

Most cheap clip-on style bike lights do not give enough illumination for riding streets at night. This is not a safety issue of having the cars see you, but the issue of you seeing the road in front of you.

The Idea

When touring I found that my 3 cell plastic army flashlight attached to the handlebars with two elastic bands worked quite well. The two large elastic bands would hold it secure and allow some limited adjustment of the beam. (handlebars in the photos are triple taped for comfort, but also helps holding the light).

The Problem

For riding at night on purpose, I needed a more powerful light to see the road clearly out to a good 50 feet. Several companies have come up with, in general, expensive bike lights. Nite Hawk is a common brand. These are really nice, have built in chargers for the batteries, and give up to 20 watts of light or so. But they are in general over $100 or more, and some types are several hundred dollars. My problem is being cheap.


Then while browsing one day in 'Home Depot' I was looking for a cheap12 volt designer (home type accent lighting) fixture that I could modify to put on my bike. Nothing looked suitable, but what struck me was the size of the small bulbs. Quickly I measured the bulb and went to the flashlight section and measured the flashlights. Looked close (blister packs prevent close measuring). So I purchased both flood and spot pattern bulbs and checked it out in a Mag-Lite flashlight at home. Voila!

The Solution

Click any image for larger image

The result is the cheap 20 watt bike light. I found that the spot pattern is best for biking and the flood pattern was better for a car light. I also found that a standard sealed lead acid battery (Gel Cell type) would fit in a standard water bottle cage if the upper lip was cut off. So now I had the same power for a fraction of the price. Specifically, 20 watts for several hours, depending on your battery size, only costing from about $40 to $60. This conversion also makes a really nice general purpose 20 watt flashlight. It will be about 10 times brighter than a regular flashlight.

The “How To” Section

Required Items

Gel Cell type sealed battery, 12 volt, 6.5 Amp Hrs, $6 -$24

Mag-Lite C cell type flashlight, $14

20 watt, 12 volt sealed halogen designer spot light, $5-$8

small microswitch, $3

Car cigarette lighter extension cord, $5

2 crimp terminals that fit the battery terminals

Heavy bike water bottle cage, $9

Attaching the Light to the Bike

For my bikes I have bar-end shifters which allow me to have the light up next to the brake levers. Bikes that have the combination shift/brake STI type system will need to mount the light head back farther. Depending on your specific setup, you may have to use your imagination on mounting the light. Since the only part of the flashlight really needed is the head assembly, some of the barrel could be cut off and used in a bracket arrangement bolted to the handlebar.

Another Mounting Suggestion

With this method a block of polyethylene foam (a stiff, slippery closed cell foam), commonly used in packing, is carved with a V groove in the top and a round (ish) groove cut at a right angle to the top groove. The round groove has a piece of rubber bike inner tube glued in the bottom area (use rubber contact cement) to give more friction when mounted on the handlebar.

A section of inner tube (from a car tire repair) is used to loop over the front of the flashlight, under the handlebar, and over the back of the light, producing the above mounting. This method is useful for STI shifters and mountain bikes. Innertube makes a good stiff elastic and I prefer it to normal elastic bands.

Attaching the Battery to the Bike

The sealed acid battery fits nicely in the water bottle cage and is attached with a couple of heavy elastic bands from sections of an old car tire inner tube. Some bikes have two bottle cages and there is interference installing the battery. You may have to remove the other cage or improvise. Since the system has no recharger, don't fasten it on permanently. The power cord is run to the front of the bike using velcro (celery bands, no less) being sure to leave enough slack for turning the handlebars.

Other Conversions, Variations and Safety Suggestions

Note that a 20 watts will generate a lot of heat and the head will get quite hot, (especially with the Philips bulb) so I don't recommend converting a plastic flashlight for continual use. But it can be done, as I have a $2 plastic flashlight that I converted to use with the extra GE flood pattern halogen bulb. The wider beam is perfect for an occasional car light that plugs into the cigarette lighter outlet.
A Stripped Down Version of the Mag-Lite Conversion
This version of the Mag-Lite uses just the head of the flashlight, thus reducing the complexity and weight. You don't have to struggle to remove the internals of the Mag-Lite. Which have changed slightly over the years and models. So the previous directions might not be exactly apply. This new version uses a plastic spring clamp to hold it to the handlebars. It uses the same bulbs and battery as used for the full flashlight conversion. This section shows how the conversion is done.

Kayak Light

Having purchased the Philips bulb I decided to make a waterproof flashlight for kayaking (salt water). I used another Mag-Lite C flashlight, a plastic Rubbermaid food container, a standard ¼ “ computer power cord, two brass Swag Lock compression fittings, three TruSeal nuts (to backup the pipe thread on the compression fittings), and another 6.5 AmpHr battery, Cheap waterproof penetrations are constructed by using SwagLock tubing compression fittings that are enlarged slightly by drilling to allow the power cord to pass thru. The cord is sealed with a wrapping of teflon tape under the compression nut. The battery fits nicely with foam inserts and does not move around. For this project I did not remove the switch on the Mag-Lite. The switch has a thin piece of rubber taped over the real switch. The original switch button is not waterproof when submerged several feet. When reassembling the flashlight, use the original plastic lense cover in front of the halogen bulb in case water is splashed on the front of the hot halogen bulb while paddling. I used a plug of wood as a dummy battery and wired the power cord thru the bottom of the flashlight. The battery container is stored in the kayak cockpit and the flashlight is slipped under the elastic deck cords on the kayak. The 36 degree beam is broad enough to illuminate a wide area out 50-100 feet.
Cheap Helmet Lights and Others
Helmet lights are useful because they allow you to see your way around curves at night and look at anything you turn your head toward. Good for glancing at car drivers, to get their attention when you are not sure they have noticed you. Some use them instead of bike mounted lights. Many commercial models are available, but sometimes you just want to adapt an existing light. A few good ideas using AA lights. This section shows some ways of doing it cheaply.
Cheap Bike Reflectors
While all bikes come with pedal reflectors (clip-on pedals don't) and a rear red reflector and a white front reflector, this is hardly enough to really be seen on the road night. The current generation of flashing red lights are very good and very effective but reflectors are nice. For ideas to increase your visibility with reflectors see this section.
The Bike Prop and Brake Locks
Ever lean your bike against a tree or pole and have it slide forward or back and fall over. Ever had difficulty packing you panniers or BOB trailer in a campground with the bike propped against a picnic table or tree. This section solves all that for you.
Dual Mirrors
A mirror on both sides of the bike just makes sense. This is what I did to solve that problem.
Additional Rear Flasher hanger
I have two red rear flashers on my bikes. One on the seat post and one on the rear of the rack. My bike bag obscures the flasher on the seat post. Here is how I solved the problem.
Bike Light Brightness Comparisons
After looking at all the bike light options offered on this site, you might want to see the differences in the brightness of the lights. You might not want to trust that new LED headlamp after looking at this comparison. To see the difference between the Cateye Opticube 5 LED, Lowes 1 watt LED light, the MadMax AA LED conversions, and the 20 watt 'cheap' halogen light, link to this section.
For more information on bicycle headlights, chargers, mounting, and more, check this reference. From the UK, bike light resources. For information on building your own and examples check this list. You can also go the the Bike Forums and do a search on bike lights. This mountain bike forum has information on building your own.