I run most, if not all, of my projects on 3V because of cheap and small batteries. Both 2 x 1.5V or a small 3V coin cell. This little project was done in an hour and turned out to be quite a helper. I use mostly Arduino Pro Minis when setting up and testing circuits, but this testing board can of course be used with anything running on 3V.
It's intended to be used on a breadboard for easy testing without having the FTDI adapter connected. It serves two purposes. First, I can move the project board to where it is supposed to be located when finished. That's a must when testing for example radio range or reading temperature measurements. Second, I'm able to measure the current used when running on battery, and thereby able to calculate battery life.
I made a vero board strip about as wide as the breadboard and glued 3 male headers to the vero board so they would fit on the breadboard. Then I soldered male and female headers on the side of the vero board for easy connection of battery terminals. The switch is essentials as you will have to turn it off when programming the Arduino Pro Mini with the FTDI adapter.
The battery can be any type you like. I use a 900 mAh CR2477 battery with a corresponding holder.
If you look closely on the top image you can see I have two small wire loops a bit up from the board, on opposite sides of the switch. This is for connecting an ampere meter when measuring the circuit. It's also convenient if I want to measure the actual voltage.
A piece of vero board
Strips of male and female headers
A battery holder