Summary:
We can use an Arduino Uno board as a power source. It will transform the geometry of the power barrel jack or the USB connector into sockets we can use with jumper cables. These will easily plug into and power solderless breadboard projects.
When using an external 9 V supply, we have access to the raw 9 V supply on the Vin pin, the 5 V pin connected to the on-board Low Drop Out (LDO) regulator and the 3.3 V pin connected to the 3.3 V LDO. These two regulators offer a precision voltage source accurate to better than 1%.
When powered by the USB 5 V supply, the 5 V in pin is the raw USB 5 V power, but the 3.3 V pin is the accurate LDO output. The Vin pin is connected to the USB 5 V through some circuitry and should not be used.
When both supplies are plugged in, the Vin pin connects to the 9 V raw voltage, the 5 V pin connects to the raw USB voltage and the 3.3 V connects to the on-board LDO.
Links:
In this episode, I used four components. Here are links to where I purchased them. Note: I have no affiliation with these vendors, I just like these parts:
