my first relay project and need to know about product please

by auspicious on Tue Jan 01, 2013 6:09 pm

HI. I have been looking into using a relay with arduino uno. I will need to control dc (say 12vlts)... and then also AC (not sure on amount yet, but lets assume its for some basic banned).

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.


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 ?

IN any event this is the link that first turned me onto the relay idea. Is it ok?

http://www.glacialwanderer.com/hobbyrobotics/?p=9


So, really I am just looking for the products that can get me going on this. I intend to learn along the way. Any advice is more appreciated then you could know :) !
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Re: my first relay project and need to know about product please

by adafruit_support_mike on Tue Jan 01, 2013 9:06 pm

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 warranty, 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.
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Re: my first relay project and need to know about product please

by john444 on Tue Jan 01, 2013 11:39 pm

Hi Auspicious,

I definitely agree with Mr Stone about the 120-Vac and relay.
Just don't try to make your own. Bad things can happen to good people
when they make trivial mistakes.

Adafruit does have this item:
powerswitchtail2.jpg
powerswitchtail2.jpg (4.01 KiB) Viewed 413 times

It has the opto-isolator and the relay safely inside a box with
power cord, strain-relief, indicator LED, and most important - isolation.
Unlike trying to make one of these yourself, it should be no more
dangerous than any other 120-Vac banned product.

Regarding the 12-V side of things; it depends a lot on how much
current, is it ac or dc, switch the high-side and does it need to be isolated.
If dc and under about 100-mA, low-side switching, non-isolated a transistor will do.
few amps and does not need to be isolated, a power FET may do.
With higher current, AC, isolation or high-side switching, a relay is probably needed.
(you can usually operate a relay coil using a transistor or FET)

You did not say what you were doing so all the above is a generalization.
YMMV
Good Luck, John
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Re: my first relay project and need to know about product please

by auspicious on Wed Jan 02, 2013 4:31 am

Excellent. Thanks to both of you, I will not be burning my house down this year. (and replacing batteries in my smoke detectors :) )

Ok, so it looks like I will be buying those prebuilt relays for now, and learning more before moving on. I am controlling heating element.HOwever that can get expensive, so i will need to learn quickly.

thanks again.
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