Well the replacement regulator datasheet says that pin 5 its not connected to anything so I'm assuming that it doesn't matter. With regard to the PCB, I don't know the answer to that and didn't really consider that. I don't have the actual schematic but my understanding is that all Arduino Mini Pro's are made from the same schematic so if so, pin 4 per the schematic is "bypass" and is not attached to anything on the PCB.

@thecricketer There are so many things to consider when selecting components but for us hobby-tinkerers there are a few that are most important e.g. Max input voltage: Vinmax Max output current: Iout Size or package Price You will find all these answers in the data sheets. Normally available from your components supplier or else on the internet. Below is a simple table: Type Vinmax Iout Package Price (SEK, single units) LD1117 15 V 800 mA TO220 5:50 LE33 20 V 100 mA TO92 9:- LP2950 30 V 100 mA TO92 9:- LM317 40V delta 1500 mA TO220 9:- (There is also an 100 mA version for 6:25) If you are using the Sundberg newbiePCB the design is for an TO92 package so either LE33 or LP2950 is the choice. The LD1177 is TO220 package and you do not need 800 mA for the radio. It only consumes about 30 mA when transmitting. The LM317 is an adjustable regulator so it can regulate to 5 V or 3.3 V or whatever you like but it takes two more resistors to set the voltage. In this case it is overkill and does not fit the Sundberg board. If you plan to run your design on batteries I think the LP2950 is better than the LE33 because the LP2950 consumes less power internally, only 75 microamps. The LE33 can use as much as 500 microamps internally. This is just scratching the surface of component choice but keep it simple in the beginning. Happy tinkering!

@thecricketer some ideas here https://www.youtube.com/watch?v=FqtNsIKpZAw

Welcome to the MySensors commmunity @thecricketer! You would probably be better off minimizing power consumption of all components (see https://www.mysensors.org/build/battery for some recommendations and post a list of your components for more specific advice) but yes, what you are suggesting is possible. Easiest way: use 2 2xAA battery holders. Connect + (red wire) of both holders to Vcc on the device and - (black wire) of both holders to gnd on the device. Done.