If you need to save size and weight an Atmega 328 chip on a piece of stripboard will run off 2 x AA alkaline cells with its internal 8MHz oscillator. That way you avoid a lot of irrelevant power consuming parts including the energy wasted in voltage reduction.
An Attiny 1634 has nearly as much horsepower as an Atmega 328 and comes in a much smaller package that is still hand-solderable with care.
...R