Originally Posted by rhm

To retrofit an old headlight housing to run an LED, you have to fit the LED together with its own lens (which is a small acrylic parabolic reflecto). The LED comes on a hexagonal aluminum plate about 2 cm across, and the lens is in a cylinder about 2 cm long that fits onto that. You may be able to fit that LED and reflector combination into the parabolic reflector of an old lamp, or maybe not. It depends how the light bulb was attached to the parabola. You can get narrow, medium or wide lenses for most LED's now, and a narrow one provides very well focused light that is ideal for bicycle headlights.

Aside from fitting the LED and its lens, there is the matter of a heat sink. An LED produces heat and light at the same time, in the same place. It isn't a great deal of heat, but it is pretty intense at that point. The aluminum hexagon will disperse it to some degree, but you also want to attach that aluminum hexagon to another, bigger, piece of metal to disperse the heat farther. One way to do this is to replace the original parabolic reflector of your lamp with a disc of sheet copper, and attach the LED, with its reflector, to that. The copper disc mounts to the lamp with the same steel springs that held the original parabola, so the heat produced is transferred first to the copper and then to the lamp housing, which has enough mass that you won't even notice it getting warm.

As for the electronics, the fundamental challenge is that a dynamo produces AC power and an LED requires DC power. You can put a bridge rectifier in the circuit (this requires four soldered connections) or assemble your own from diodes (this also requires four soldered connections, so it isn't actually any more work than using a pre-assembled unit). Or you can go caveman and wire your headlight and tail light opposite; so one wire from the dynamo goes to the positive connection of the headlight and the negative of the taillight, while the other goes to the negative of the headlight and the positive of the taillight. As you ride, they will flash alternately.