With a DMM attached to the output posts to accurately set the voltage one could also power an Arduino or other small project. Using a 9 volt dc battery connected to a 2.1mm barrel jack it's small enough to fit in yer pocket & so totally portable.
I consider this a fairly simple & fun project leading to a totally useful & versatile tool when complete. Soldering on the breadboard pcb is a bit challenging but a decent iron with small tip makes short work of it.
With 9 volts dc provided by wallwart or battery a project can be supplied with between 1.25 - 8 volts dc by connecting to 2 posts with alligator clips or bare wire. A toggle is included for on/off with an led to show when powered up and a knurled knob for voltage adjustment. All electrical wiring & components safely contained within an Altoids tin.
After making who knows how many drawings attempting to fit this circuit into the tin I found putting it down on graph paper allowed me to see where everything goes quite readily. I've just drawn enuf to get the gist of it. It is a regular breadboard after all with double power rails top & bottom (tho I have only used the negative rail) & 2 solid rows of 5 holes high each.
Is Texas Instruments datasheet. You'll find an almost identical schematic when googling Nat Semiconductor, Fairchild, et al. This is the basis of my design. Please note the R1 resistor here is 240 ohm, I've replaced it with a 470 ohm for a reason I've now forgotten but I do remember reading somewhere this value should be 100 - 500 ohm.
The 3rd image is a page of my design notes showing the late stages of how I arrived at this particular configuration. I've actually been working on this for over a year, off & on, with the project only coming together during this past month.