Making life easier… (13) Upgrade your dynamo lights

It’s a good time of year to make sure your bike lights continue working. Dynamo lighting systems are cheap and effective, as well as being less attractive to thieves. This month David Green describes how to upgrade a dynamo system to a twin-cable system to make it more reliable.

Most ‘dynamo’ systems rely on the bicycle’s frame as part of the electrical circuit between the generator and the light. If you look at a bike with dynamo lights, you can see that there is only one wire going from the generator to each lamp. I call this a ‘single-cable’ system. To complete the electrical circuit, there needs to be an unbroken ‘return’ path from lamp to generator as well. In single-cable systems, the steel or aluminium bike frame and its components serve this purpose. Unfortunately, it takes only one poor metal-to-metal connection somewhere in this return path to stop a single-cable system in its tracks. (If you have ever had the slow and frustrating job of locating a rusty connection, you will know what I mean!)

My solution is to bypass this inherent weakness by wiring the generator as a ‘twin-cable’ system with one wire for the ‘live’ part of the circuit to each light, and one wire acting as the ‘earth’ part. Anyone who is patient and happy to wield a soldering iron will find this upgrade easy and worthwhile.

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What you need: [A] soldering iron, [B] loudspeaker cable, [C] cable ties, [D] round connectors, [E] heat shrink plastic, [F] lamps and dynamo.

Tools and materials

The following items can all be found in hi-fi, DIY or electrical stores. (I went to Maplin and Mackays).

  • Soldering iron (plus flux and solder).
  • Sharp knife (to cut and ‘strip’ cables).
  • Twin-core ‘loudspeaker cable’ approx. 3 metres. Make sure you can distinguish each core.
  • Cable ties in 10 cm and 15 cm lengths. Get about ten of each.
  • Round connectors. Get four of these with holes big enough to accept the bolts used to mount your front and rear lamps, and your dynamo. (I needed both 5 mm and 6 mm diameter holes.)
  • ‘Heat shrink’ plastic and a hot air gun or similar (optional).
  • Front and rear lights and a dynamo. I used a standard ‘bottle’ dynamo on my rear wheel, a halogen front lamp and a busch&müller DToplight at the back.

Getting ready

Allow about two hours. Try to find a workspace with good lighting and where you can sit down to do the fiddly soldering jobs. Before you begin, make sure that your lamps and dynamo are positioned effectively on your bike. See the article on Lighting in Newsletter 32.

What to do

Follow the sequence described in the following photos.

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1. On the front light, identify the ‘live’ and ‘earth’ wire attachment points. My light has a small screw for clamping the live wire (+). The earth wire will simply attach to the lamp bracket (-). 2. Better quality lights are often designed with two-cable systems in mind. My rear busch&müller DToplight has separate terminals for the live wire (+) and the earth wire (-). On other models, the terminals may not be so obvious, but you can usually work them out by ‘working backwards’ from where the bulb contacts are.
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3. For each light, plan how you can best route a cable back to the dynamo, then cut a length of the twin-core speaker cable about 50 mm longer than you need for the route. Decide which of the twin cores you are using as the ‘earth’ core. (Note: you must use the same core for ‘earth’ at both ends of the cable.) Start by preparing the connections at the lamp end of each cable by stripping about 10 mm of the plastic from each core. You might need a ‘push-on’ connector [A] or a ’round connector’ [B]. ‘Bare wires’ [C] are more durable and easier to handle if you apply solder to them. 4. To start with, just prepare the connections you need at the lamp end of the front- and rear light cables. For a really neat finish, before you attach a connector, slip a piece of heat shrink plastic onto the cable.


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5. Attach the cables to each lamp and re-check your cable routing all the way back to the dynamo. You may need to trim the length of each cable so that it reaches the dynamo neatly. In this picture you can see where the small screw on my front lamp is clamping the live wire (+) while the earth wire (-) is clamped to the lamp bracket. 6. Taking care to distinguish the ‘earth’ wire from the ‘live’ wire, solder the necessary connectors to the dynamo end of the cables. Now connect up at the dynamo. In this picture you can see the round connectors I fitted to the earth wires () from each cable clamped to the same bracket attachment point.


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7. On my bike the ‘live’ wires from each light cable are attached to the dynamo as ‘bare wires.’ 8. Use cable ties to secure the cables neatly against the frame. Make sure that you do not accidentally trap any gear control inner cables!


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9. Trim excess cable tie neatly for a really professional finish.