Instructables
Instructables has lots of DIY bike lights, but here is my version which is cheaper and lighter than most. The difference is that I use smaller MR11 halogen bulbs that results in a more compact product, at the expense of somewhat reduced light output and efficiency. Since this is a pretty simple project and there are tons of other resources with more detailed step-by-steps I will just provide the basic components required and some pics to make the whole thing easy to understand.
 
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Step 1: Bill of Materials

As previous makers have noted there are a wide number of parts and bits you can use to complete a DIY bike light project. The most important are the bulb, connectors and the switch. Since I tend to commute rain or shine I chose connectors and switches that can handle water. Sealed bulbs with front glass are also required. The connectors are "trailer" connectors you can pick up at the local auto parts store, and the switches are waterproof units purchased conveniently from the same source as the bulbs and other parts, www.batteryspace.com.

1. MR11 Sealed Halogen bulb
I suggest a 12V 15W spot for a good all-around light that will illuminate your path well. If you are more concerned about lifetime, you can get by with a 12V 10W spot. Stay away from the floods if possible.
Sources:
12V 15W Sealed Spot
12V 10W Sealed Spot

2. PVC Body Parts
I have not had any trouble with the regular white PVC failing or melting at up to 20W. Over volting a 20W from 12V to 15V for more light output may cause softening of the PVC since you are effectively upping the wattage from 20 to 31W.
Sources: You can get these parts at any home store such as Home Depot, Lowes, Menards, Do-it-Best, etc. Look for 1" PVC couplings and misc parts. You are looking for 1" threaded FIP cap and 1" threaded male adapter. See pic below for an idea of what you will need.

3. Bulb socket.
You can probably just solder on wires to the bulb but the correct sockets are available from batteryspace.com for around $1USD so why not just get it.
Source: Ceramic MR11/MR16 Bulb Socket

4. Trailer connector
It is best to have a simple way to disconnect your light from the wiring harness of your bike which includes the switches and battery. I choose to use 2-wire automotive trailer connectors which are designed to handle large currents and operate in wet and dirty conditions. See pic below to get an idea.
Source: 2-wire trailer connector

5. Metal mount.
You can use whatever you have handy to make the mount. Since I wanted a light and simple setup, I used a single piece of light stainless sheet I had laying around. Bent it into a clamp and added a bolt through the rear PVC cap as pictured. A bit of rubber cut from an old innertube will protect your paint from the clamp.
Source: Whatever you have around.

6. Waterproof switch.
I have the switch permanently mounted to my bike between the trailer connector to my light, and another trailer connector to my battery. This allows removal of the light and battery as needed and the switch and wires can be cable tied, duct taped, or otherwise mounted.
Source: Waterproof switch

7. Battery
Of course you will also need a suitable battery. These halogen bulbs are rated for 12V but you can run them at slightly higher voltage to get a lot more light at the expense of reduced bulb life. Many people run halogens at 13.2 or 14.4 volts depending on their choice of battery, NiMH or Li-Ion. The building of battery packs is a detailed topic in itself and can be found discussed in great detail elsewhere. For battery cells or pre-built packs, www.batteryspace.com is a great starting point.
Gazbert4 years ago
I can't find anything like this in the UK. Has anyone else?

2-wire trailer connector
ComradeLynx5 years ago
Is that a Motorola PMR mobile radio power connector you're using?
tjoe5 years ago
what about the battery what voltage batt does it take? p.s. could i use a 9v energizer?
jmengel (author)  tjoe5 years ago
The bulbs are designated as 12V. A 15W bulb at 12V will draw 1.25 amps. Often, people overdrive these 12V halogens at ~14V (and thus more current) to get more light. The lumens per watt increases somewhat for these lights when overdriven, at the expense of reduced lifespan. Underdriving increases lifespan significantly but the efficiency and light output are reduced. So to answer your question, I would recommend at least a 12V battery, with at least 2.5 amp-hour capacity to allow for ~2 hours of runtime. Using a 9V would not be satisfactory. A typical 9V battery has less than 300 milliamp hours of capacity at high current draw. The bulb would draw ~900mA and deplete the battery in less than 20 minutes, not to mention the fact that it would be very dim.
SkinnE7 years ago
I like the compactness of your design vs. the other lights on here. They are all improvements on what I've seen before, but look too bulky. I'm lamenting the fact that I no longer work in a bike shop, so a commercial system to replace my old set up is a bit out of my budget. I'll be trying this one soon, but will probably vary the mounting.
I've read that these conduit hangers are perfect for mounting things to bikes.

http://www.lowes.com/lowes/lkn?action=productDetail&productId=75308-15527-49110&lpage=none

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