The Boost Shield Kit allows you to use supply voltages under 5v with the Digispark development board.
The Boost Shield boosts a 2-4.5v supply (including 2 aa or aaa batteries, or on lipoly) to 5v and connects directly to the Digispark.
This is an unassembled kit and requires basic soldering.
This is designed for use with the Digispark development board, which is not included.
Schematic design of Boost Shield Kit
Instruction and Tutorials of Boost Shield Kit