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Powering Projects from a 1.5V Battery
technoblogy.com
A single AA or AAA battery would be an ideal low-cost power source for a small AVR-based project, but unfortunately they only provide 1.2 to 1.5V, and even the low-voltage versions of the AVR chips need at least a 1.8V supply.
The solution is to use a simple voltage-doubler circuit to step up the voltage. The voltage doubler uses an oscillator to charge up two capacitors through diodes. The voltages across the capacitors are effectively put in series, creating double the supply voltage.