I found the idea and the schematics on the Internet, but now I am unable to find the web site, although I searched a lot. Perhaps it does not exist any more.
Instead of 4 rectifying diodes, I use only 2 diodes and 2 capacitors, and this schematic doubles the output voltage, which is very useful for the LEDs. Schottky diodes are used because their voltage drop is very small.
Here are some old pictures, unfortunately I don't have many:
Before that I glued one side with aluminum foil, to reflect some light.
I used 2 very small metal plates to solder and extend the thin cables that come out of the dynamo, because if you operate a lot with them, you can easily damage them.
Here is the schematic:
created with: http://www.docircuits.com/circuit-editor
Vac0 is the dynamo.
It is possible to omit C2. In that case the LEDs will light up faster, but they will also dim faster!