I've been waiting for a post like this for a while. First, please accept my hearty congratulations. Let me say that I've built something similar for my bicycle trailer, but I've been wanting to ask the opinion of someone knowledgeable in this area.

Background: My power source is 12v dynamo + rectifier/smoothed + voltage regulator-> ~10.4v DC. I have a little box of electronics that has a voltage meter (for fun) and a voltage regulator that outputs 5v for USB devices and my 1W LED headlight (ebay thing, it has a built in current regulator on the heatsink). The original 10.4v is also fed back to some rear LED lights I made. But as per your allusion, I can't run both rear and front lights in parallel. I assume it's some sort of gross impedance mismatch.

Q1). I don't know if there's a standard solution, but do you think if I made a 555 timer 'switching circuit' that switched between rear and front lights at an imperceptibly high rate, this would be be a 'good electronic engineering solution'? If I added an adjustable switching rate I could make it act as a flasher too.

Q2). I've never been sure exactly how to rate NiMH voltages, but under load they're usually about 1.2v (1.4v open circuit, fully charged), so do you find that 4.8v reliably drives USB devices? The iPod, for example, seems to be notoriously picky about the exact pin voltages.

Q3). Ignoring the voltage drop, is there any merit in adding a diode in series with the USB? I imagine most devices have their own protection built in.

I'm sure i have photos of it somewhere.

All the best,
AR.