Trying not to make this request project specific, but I keep encountering it and not learning from my reading of stuff on the web and it never sinks in at a level when I understand.
This request is basically "how to manage a project where multiple voltages are used within the one project?"
It comes about from trying to build various Arduino projects where the MC only needs a small 3.3V, However, a good amount the sensors, breakouts boards and other "stuff"you'd want to use, require a power supply of 5v or more.
Context:
My current small project is to monitor the pool water level using the Milone E-Tape sensor.
I'm using a moteino as the low power MC via a 3.7V Li-Ion battery and this is to last a long time.
The sensor board (spec doc link) needs 6-30V DC, and I figured I'd use a 9V battery and somehow activate it's power supply via a MOSFET or something, read the sensor level and send it to the IoT gateway, and shut it all down again for sleepy time until the next read in a minute or so.
I keep getting hung up on the controller side having to deal with the "do-er" side of the things that frequently operate at higher voltages than the MC. Eg, an arduino controlling a set of servos and a range of lights / LED digit displays in a home cockpit project and needing a chunkier power supply for those servos and lights, of which would would larger than the power supplied used by the arduino.
I know there is a whole heap of 'gotchas' that need to be kept in mind for things like this (common grounds, avoiding the ardunio being backwards powered across the controlling pin) and other stuff. 
Love the vids @baldengineer! Just subscribed after stumbling across the 100K sub special - congrats!
