I have a 16 year old son who is off-and-on showing interest in learning electronics. He wants to work for NASA someday. I’ve looked at dozens of Arduino and Raspberry Pi kits but I feel like he would benefit not so much from “mostly done for him” types of kits that are more like toys, but the kind that teach more fundamentals like resistors, capacitors, transistors, relays, etc. In other words, knowing first what the principles are behind the fancier kits. Do any of you have a recommendation of kits that start with the fundamentals, but that can still be inspiring and engaging?

Well you could start by building a small USB SPI converter from an old serial port maybe.
Usually in there, there's all the basic components in one of those.

After that you could use that knowledge for starting with an Arduino UNO.

Also, if he shows interest in music you could work with high and low-pass filters and amplifiers for, maybe a custom soundboard or something.

This post was made by a 17-year old so I'm not the most proficient person I nthe field but I do work with embedded systems.

I wouldn't be so fast to say no to arduino or even the rpi.

Yeah, they are all built and ready to use. But I find it less overwhelming to learn new stuff when I know how to control at least part of that system. So you have an rpi, right? You hook connectors on your gpio. Now you can pass a signal. But where to? Will the receiver understand that signal? Isn't it too strong/too weak? Where does it go then? Do I get some kind of a response then? How do I read it? How do I shape it?

Idk about your kiddo but when I learn new things I do like to have control of some part of them and see practical applications of what I'm building while learning. RPi ticks both of them.

Arduino is lower-level so I'd use them for intermed controllers

Arduino fits nicely as the brain for projects with fundamental components.

Having a kit as a learning tool might be better than doing everything from scratch right away. Those kits and their guides are made so that you can learn step by step in, and so that there's a reward at the end of each task :)

Elegoo Mega 2560 is a kit that is probably more what you are looking for; it's got instructions and tasks, but it's more a "see what you can do and make" kit, not a single process start-to-finish make a product kit.

What about Ben Eater's 6502 kits? Gets him right down in the details on digital first principles.

Or FPGAs are very accessible now, you could look at the icestick

There's a book named "Learn electronics with Arduino" it could be a good starter for both of you.
Then he can learn some digital electronics and if he wants to go deeper he can get Digital Logic Computer's design by Morris Mano. That goes from easy gates circuits to a 1 bit ram using transistors and up to a full blown ALU using discrete components

@Gorlami Wow! BPS.space is awesome.

Thanks, everyone, for your input and suggestions. Lots to go on and research here.

I went with the Elegoo starter. Now if Amazon would just get it here. It’s already a day late.