Hi Martin,
Thanks for the answer! I'll try to find one of those DC to DC converters. btw: the thing about the parabolic reflectors, you MUST have them EXACTLY pointed at the sun; it's a lot like being pregnant, you either ARE or you're definitely NOT! It's like a 10 to 1 heat gain comparing in-focus vs. being even a small amount off. So trying to estimate the sun's position doesn't work. I thought when I hooked up the solar garden light PV that it had a snowball's chance in hell of working, but it worked perfectly with no resistors or anything. It must be that the little cell (plus about 5 feet of wire acting as a resistor) puts out just enough current to be read perfectly by the A/D converter on the analog pin. It locks on the sun and the pipes stay bathed in a blazing hot band of concentrated sunlight all day long, no problem.