auspicious wrote:Do I need separate relays for both of these? I've seen shields, but they seem to tie up the entire arduino, and I'm not sure if they control both ac and dc.
You'd need a separate relay for every power supply you control, and different relays do handle different voltages.
auspicious wrote:Then It was suggested that I try this device that Adafruit carries, that has a built in relay to an extension cord, that makes it more safe (I haven't even considered safety until now). But I am wondering how many things can hook into that device? is it just one per ?
Okay.. this is where I go all "rain on your parade": wall power can kill you and/or burn down your house while your family is sleeping. If you haven't thought about safety, you don't have the background to handle that power level safely.
That's not to say you can't learn.. safely working with wall power isn't a huge mystery, but you need to know what the risks are, what tools and techniques are appropriate, what seemingly-pointless things you should always do, and what seemingly-harmless things you should never do. It's much better to learn them with someone experienced nearby to keep an eye out for the risks you haven't learned about yet. If you know an electrician, talk to them. If you don't, look for community-college or evening classes.
The time and money you'll invest in learning to do it right isn't that large, and the value you'll get is tremendous.
I know that's probably not what you wanted to hear, but voltages above about 12v fall into the category of 'safety critical systems', like flying or driving a semi. The "just get in and go" mindset can cause all kinds of damage, and the first couple things you learn when you train to use them safely are "don't encourage people without training to do things that can hurt them" and "encourage anyone interested to learn the proper techniques."
What I'm doing now is following my own training.
When you void a product warrany, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.