A simple undertaking made with recovered assets, this device offers a good use for less than fully charged batteries since multiples of the series may deliver a useable voltage for benchtop projects. Further, they provide pure D.C., sometimes desirable for prototyping circuits that don't handle "dirty" rectified supplies very well. The portability factor is also self- evident.
Step 1: The Building Blocks
I like to save battery compartments that are well made, they offer excellent housings for a variety of cell sizes: AAA, AA, C, and D. Cutting them out of discarded devices is not too difficult, and getting them ready for service requires a minimum of time.
Step 2: The Voltage Tap
A cutout section of some saved phosphor- bronze grounding fingers will suffice quite well for the voltage selector tap. Use what you have at hand however, later it can be upgraded if needed
Step 3: Ease of Use
Selecting the desired voltage is as simple as placing the tap finger behind the appropriate cell and contact as the images show. 1.5, 3.0, 4.5, and 6.0 volts, all the way up to 9.0 with this particular build is possible.
Step 4: Parting Thoughts
Project cost was nil, these parts were from my stock collection, acquired when I do "Asset Recovery Sweeps" around my neighborhood, and shelved for just such a need.