Recently, I found a gorgeous looking bicycle rear light on eBay.
It was a bit rusty but after cleaning it the following markings on it became readable:
Unfortunately, I couldn’t find much about on Miller as a bicycle light manufacturer and the different models they used to make… If anyone knows about it feel free to leave a reply at the bottom of this post.
With a bottle dynamo fitted on your bike, the frame is the return path of the electrical circuit and you only need one piece of wire that runs form the dynamo to the lights to bring the current to the light bulbs.
With dynamo hubs, the frame is not part of the electrical circuit and you need to lay a 2-core wire from the front hub to the lights (usually the front light has a switch that the front and the rear light on & off).
It was quite easy to remove the original one core piece of wire to replace it with a 2-core one: I drilled into the rear of the light through the 1-core wire and also removed all the tar that was used to seal the unit and make the lightbulb holder stay in place.
Next step was looking into how to replace the lightbulb with an LED. LEDs on a bike are advantageous over incandescent lightbulbs because they are absolutely immune to vibrations (there is no filament to break). Incidentally they consume a lot less current but that’s not something you worry much unless you power them out of batteries.
Generators (dynamo) are AC generators. Their output tension is not regulated (the faster you cycle the more energy they produce) so I had to think about voltage and current regulation before using LEDs.
The circuit can be broken down in to the following elements
- The hub AC generator
- The diode bridge that turn AC into DC.
- The zener diode that limits the tension to 5.5V
- The 0.47F (yes half a Farad) memory backup capacitor that smoothes the DC tension.
- The 150mA / 3.3V positive voltage regulator that drop the tension and limits the current in order not to fry the LED
- A miniature 1uF capacitor to prevent the regulator from oscillating
- A 30 Ohms resistor to lower the tension down a bit before feeding the LED
The design above has the advantage to give a “stand light” facility. That is to say, when you stop cycling (at a red light for instance), the 0.47F capacitor is charged an keeps powering the circuit. Depending on how much current is draw by your LED the light should stay on few minutes after you have stopped.
Design considerations if you if you want more that one LED:
- Make sure you don’t exceed 150mA of current draw (or use regulators in parallel or a bigger one)
- Make sure there is always a resistor before each LED if you use LEDs in parallel
List of components:
- 4 x silicon rectifier diodes (more or less any type will do)
- 1 x 5W 5.5 Volts Zener Diode
- 1 x 5.5v 0.47F memory backup capacitor
- 1 x 3.3V / 150mA positive voltage regulator (TS2950CT or equivalent)
- 1 x 1 uF electrolytic capacitor
- 1 x 30 ohms 1/4 watt resistor
- 1 x red LED
It should cost you less than £10 to source the parts above. If you are based in London, the people at Cricklewood Electronics should have the components needed in stock!
Bellow are two pictures of the modified Miller 597 installed on my road bike.