I have only a cursory knowledge of electronics... I "know" what all the basic components do, but I have not figured out how to use that knowledge to design anything (much less evaluate someone else's designs). I will be solely reliant on YOUR upvotes to give me the right answer.
The Raspberry Pi is a tiny/low-power general-purpose computer (ARM) which has a set of "GPIO" pins which allow it to interface with other components. It has a 5v power pin, a ground pin, and several pins which can be set to "high" or "low" (where "high" = 3.3v) that can be used to trigger other events.
I know I saw it, but I can't recall now where (and can't seem to find it), but the max current draw for the 3.3v is fairly low... in the low-hundred mA.
The primary "chip" (processor, GPU, etc) is something from Broadcom.... and they refuse to give specs if you haven't signed an NDA.... so, WHAT may damage the computer is a pretty big question, but people have reported that "a little" current going "back up" the GPIO output pins has been enough to fry the board. I assume (but see the aforementioned "to start"), but don't REALLY know, that this means the resulting circuit may need to be opto-isolated?
Background (part 2)
Everyone in the world seems to want to use these things to control real-world objects since they're general-purpose Linux machines (and can, thus, be programmed in your-favorite-language). The problem is that all of the relay boards in this space are, currently, designed for the Arduino's 5v output.
Question I have seen a design or two for how to build a circuit that can use the 3.3v to use the 5v to trigger a relay, but there doesn't seem to be any consensus in the community for how to do so SAFELY. Given that I'm not qualified to evaluate the supplied circuits, can someone please explain to me how to use a 3.3v trigger to SAFELY use a 5v line to operate a relay?
Bonus Points: There have been people claiming success at using the various SainSmart 5v relay boards (designed for Arduino) on the 3.3v lines... is this safe? Will it ACTUALLY work?