Repairing bicycle lights

22 November 2013

Repairing a bicycle light that has stopped working is usually possible. This job is different from most other bicycle maintenance since it involves electronics and requires a different set of tools. Nevertheless it is relatively easy for anybody with a basic understanding of electronics to get a light working again.

Most battery operated bicycle lights are a simple electrical network where the batteries are connected to a light with a switch in between. Nowadays practically all lights are LEDs, but on older bicycles you can still sometimes find incandescent or halogen light bulbs.

Diagnosing the problem

If you are lucky it is simply a case of empty batteries. The first thing you should do is replace the batteries and see if that has any effect. If you have a multimeter at your disposal you can check the battery voltage. Simply set it to direct current ⎓ and connect the probes with the poles of the batteries. On many multimeters you can specify the upper range of the voltage that you are trying to measure. In this case that would be one of the lower settings, on mine it is 20 volts.

You can check the batteries individually or as a set. For 1.5 volt batteries you want a voltage somewhere between 1.0 and 1.5.

Turn the light on and see if it lights up. Check if the switch is working by measuring the resistance between the two contacts. A multimeter has a resistance setting which is usually indicated with an "Ohm" Ω symbol. You can use this to check if certain elements are conducting electricity. You can use this to check if the circuit is closed. By systematically probing the batteries, switch and light you can isolate the problem.

Fixing common problems

When any of the components are not touching where they should you can try and gently bend them so that they will touch. Sometimes this is enough to fix the problem. A small flathead screwdriver works well for this.

Often the contacts of the battery or switch have gotten dirty or started to corrode. In that case some contact spray will help clean them. Another. As a last resort you can also use a file to remove the top layer of the contact. This is however not recommended, as often the contacts are copper plated and you are filing off the copper.

If you have ruled out all the other components the light itself might be broken. Incandescent or halogen light bulbs are burn out over time and are usually easy to replace. LEDs normally don't burn out. If they do you probably have a badly designed light that outputs way too much power than what the LED was built for. If your light is powered by a dynamo it might lack the proper circuitry to control the voltage.

If you have bad light you might want to replace the whole thing. It is usually possible to desolder and replace the LEDs, and even add a resistor of your own, but unless you know exactly what you are doing the problem will likely come back.

Dynamo

There are several types of dynamos. They all have a rotating part and two electrodes to output the power. If you have the kind that rubs against the side of the tire it will probably break down after a few years. If it is worn down it will give a lot of resistance while rotating. My advice is to just remove the whole thing and not bother replacing it. Just get some battery powered lights, they are cheaper, more reliable and don't add any extra resistance while riding.

Wiring

Dynamos are connected to the light with some wires, this is another weak point that often fails. Some (older) bicycles use the fact that the bicycle frame (and fenders) are conductive as a way to close the circuit. In that case there is only one wire running to the light.

If you have a broken wire it is usually easiest to replace the whole thing. To prevent the wire breaking you can make it into a spiral around where the front fork connects to the frame.

To fix a broken wire you need to cut it open, strip off the insulation on both ends. Add some heat-shrink tubing, then solder the pieces back together (or add a piece in between) and use a heat gun to apply the heat-shrink tubing.

Do not solder in parts where the wire is bend or has to flex, soldering tin is not flexible once it has cooled down and will break under stress.

Useful links

• How to use a multimeter