Doing low energy application needs some special complnents and code-lines care. Some components give this feature, some other need to be worked on in short time. the main idea when we work in very low energy application is the type of battery.
We are going to use TPS6122x from texas instrument. it provides regulated power-supply solution for products powered by either a single-cell, two-cell, or three-cell alkaline, NiCd or NiMH, or one-cell Li-Ion or Li-polymer battery. it operates by input voltage from 0.7 to 5.5 v and gives stable output voltage. it exists 3 versions :

- TPS61220 : adjustable version, you can fix the Output Voltage from 1.8 V to 6 V

- TPS61221 : 3.3V fixed output, used in this instructable.

- TPS61222 : 5.0V fixed voltage

it has good efficiency with low quiescent current : 0.5 μA. and low consumption current in shut down state : 0.5 μA.

it is a good choice for a long life-time and can assure a voltage stability.