I'm running into an issue when I tried to build this: http://www.instructables.com/id/20-minutes-USB-solar-charger/. I'm using a 9v solar panel. As soon as I connect the ground from the lm7805 to the ground of the circuit, my multimeter reads 0 volts on the output pin on the lm7805. Before I connect the ground, it reads about 9 volts. I even replaced the lm7805 thinking something was wrong with it. I think I'm not understanding something simple, but I don't know what it is. I'd appreciate some insight into this issue I'm having.
One possibility I'm wondering about is if the lm7805 requires a minimum current, and the solar panel isn't meeting some minimum current requirement. Is that even a possibility?