It is more common that we can use a portable USB power supply for charging up mobile phones. I know that Arduino accepts USB power as its power supply. By reading the schematic, I found that there is no voltage regulator for USB power supply.
My question is, how can I make a regulator for input voltage 4-5V and output voltage is more or less 5.0V for arduino? Or can we buy directly from eBay or somewhere else?
It is useful as I have one USB phone charger.
What you ask is SEPIC or Buck–Boost dc converter, Google it.
Buck (Step-down) - The output voltage is lower than the input voltage
Boost (Step-up) - The output voltage is higher than the input voltage
SEPIC - The output voltage can be lower or higher than the input voltage or equal to input voltage
Buck–Boost converter - The output voltage is of the same polarity of the input, and can be lower or higher than the input or equal to input.
or u could make up u own one, use 1 Buck + 1 Boost or 1 Boost + 1 Buck or isolated ?uk or -5V + ?uk converter ...
electronics is art.