Just a quick note on battery life.
When putting together a circuit where you need to know how long it will run on a battery, you need to know how many amps the circuit draws. Once you have that, you need to know how many amp-hours a battery is rated for.
For example, if your circuit draws 1 amp of power and you have a battery that is rated for 1 amp-hour, that battery will run the circuit for about one hour.
Wikipedia has a list of batteries and their typical amp-hour ratings at http://en.wikipedia.org/wiki/List_of_battery_sizes
What I found surprising was that a typical 9-volt battery is rated for 565 mA/hr (milliamps, not amps), while a typical alkaline AA battery is rated for 2.7 A/hr.
However, a word of caution: circuits also require VOLTS. Therefore, if your circuit pulls 270 mA/hr but requires 3 volts, you will still need two AA batteries, as each battery is roughly 1.5 volts.
At this point, I am assuming batteries in series have the same A/hr rating, but that's a wild guess at this point
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