Re: Run rear light off same dynamo as front
The nice thing about home built bike lights is that you can design them to your preferences. The disadvantage of that is that you have to know enough about electronics to take care of the problems that might pop up.
The two primary options that you have are whether you want to wire the taillight in series with the headlight or in parallel. The commercial bike dynamo lights use the parallel arrangement, which has the advantage of not losing both lights if one of the wires opens up. With a parallel arrangement, the taillight will receive AC power, so there will need to be someway to use this power. A bridge rectifier would be the most common method. You'll also need to regulate the power to the red emitter too. A resistor might be the easiest technique. When choosing the resistor value, you'll want to remember that whatever current you put through the taillight is current that is being taken away from the headlight. Also consider that since your headlight will have about 9v across it, you'll have to drop a lot of voltage across the resistor in order to limit the current to the red Cree. You might consider using two red Crees in series in order to make the best use of the power being delivered to the taillight.
If you choose to wire the taillight in series with the headlight, the headlight and taillight will receive the same amount of current, so the red Cree will need to be able to handle 500mA. The taillight will still be getting AC power, so it will need to be able to pass current in both directions. A bridge rectifier would work, but will add more voltage drop to the headlight/taillight combination. An alternate method would be to wire a plain 1N4001 diode across the red Cree, anode to cathode, so the LED will carry the positive dynamo current and the plain diode will carry the negative current. Or use another Cree LED instead of the plain diode.... both will work.
The downside of wiring the headlight and taillight in series is that you've increased the voltage that the dynamo must produce before current starts to flow. This means you'll have to be riding faster before the lights start to work.
This isn't a short answer, but is a quick glimpse into the options that you have.