Hi, suppose that I posted a project code I've made myself for arduino and I explicitly wrote that this is my own copyright and nobody can make a commercial project by copying my code, is that true? Is it true than if someone sell devices copyed from the code I post he his violating international copyright laws because I clearly stated that I owb the copyright and that commercial product is unhautorized? Or I am wrong thinking that?
Thank you for your advice.
Can I took someone else arduino code that is not explicitly copyrighted and sell products out of it? Or it is implicitly admitted rhat the code is copyrighted to the person who wrote it and cant make products out of it?
I'm guessing that the answer/s to that depend on the countries all the involved parties are in, so perhaps you should give some details of that.
And you do have to be careful.
The temptation is to think that you are very clever and no-one else could have generated the code that you have.
Often it it the idea that is clever (patent, not copyright, and every bit as difficult to protect). While it can be reverse-engineered, it can also be re-engineered. And someone else may even do a better job. 
Astronometria:
Can I took someone else arduino code that is not explicitly copyrighted
The only thing you can be certain of is if you have documentation that explicitly says that you MAY use the code.
AFAIK copyright exists automatically - that is certainly the case for books and the like.
As @Delta_G has said the real issue is the cost of enforcing your rights. I find it much easier to sleep knowing that I don't care if someone copies any code I have created.
...R
Copyright is nothing you have to claim, it just exists.
If someone copies a code snippet 1:1 into a commercial project, chances are that that piece of code is very generic (like, using a library as it was intended to use)
If it wasn't, the commercial programmer would have to rewrite it, which usually makes it unrecognizable as you code.
Anyways, the sheer implementation of an idea is hard to protect once it's publicly available.
If you think your code is that valuable, don't post it.
If you are lucky, a big company will copy your code (or patented idea) and after 10 years or more in court you get a big payoff! 
And if you have a solid legal case you don't have to pay the lawyers in advance, they just take 40% of the settlement at the end.
Companies hate to pay for patents or copyrights so they probably won't buy the idea from you and you are more likely to get paid if they steal it and you can sue them! Unless you are successful enough that they want to buy your company... They buy companies more often than they buy ideas.or code
You can also consider yourself lucky if you are successful enough that a company wants to steal your intellectual property! 
Most "ideas" or "small products" are simply ignored except perhaps by hobbyists.
In most cases it's not hard to [u]reverse-engineer[/u] software (or hardware). Reverse engineering and copying the functionality of your code is NOT a copyright violation, but it can be a patent violation.
...Several years ago the owners of UNIX sued IBM for stealing UNIX code and using it in Linux and IBM's AIX. Eventually, the case was thrown-out and IBM won.
Astronometria:
Hi, suppose that I posted a project code I've made myself for arduino and I explicitly wrote that this is my own copyright and nobody can make a commercial project by copying my code, is that true? Is it true than if someone sell devices copyed from the code I post he his violating international copyright laws because I clearly stated that I owb the copyright and that commercial product is unhautorized? Or I am wrong thinking that?
Thank you for your advice.
Anyone can add the words "copyright" to any code. That doesn't make them owners of the code. And you can never defend ownership that way in court.
What you HAVE to do is register the copy in the appropriate agency in your country. Then you have established a record of your ownership. However, if you subsequently change something in the code, you must go through the exercise again.
Paul
Paul_KD7HB:
What you HAVE to do is register the copy in the appropriate agency in your country. Then you have established a record of your ownership.
I can see how that is a sensible practical step but I wonder is it legally necessary?
I know it would be difficult to establish copyright without some evidence, but I think (in theory at least) you own the copyright even though you have not registered it anywhere.
On the other hand unless the agency does a thorough search to verify your entitlement (as Patent offices do) the fact that you have registered something does not mean very much if someone challenges your rights.
The attraction of Open Source software is that everyone is entitled to copy it. The focus of most Open Source licenses is to stop a third party from imposing restrictions that prevent others from copying it.
...R
Delta_G:
The registration date serves as proof positive.There is the old "poor man's copyright"
These things prove a date. They do not prove that you own the copyright of the thing the date refers to.
Where it can be useful is in a negative sense wrt patents. If you can prove that something existed on date X which was before the date claimed in another person's claim for a patent for that same thing then the patent application cannot succeed. But in that case all that anyone is concerned with is what I believe is referred to as "prior art" - not ownership.
...R
The simple answer is you wouldn’t post any code on here that you didn’t want copying, then it’s pretty safe .Someone else can recreate what your software does ,but it’s more likely the concept of what you are doing that can be protected as others have said rather than code itself.
hammy:
,but it’s more likely the concept of what you are doing that can be protected as others have said rather than code itself.
I think you would need to patent the concept (assuming that is possible). Otherwise someone could reverse engineer the idea (think Macintosh and Windows) without infringing your copyright.
You can make money with a "thing" (hardware or software) that other people want and are incapable of (or too lazy to) make for themselves. If you offer the thing for sale at a sufficiently low price it may not even be worth anyone's time to try to duplicate it, even by using your code. Always keep in mind that the marginal cost of another copy of a program is almost zero.
The other thing that people pay for is after sales service where that matters. That's why Siemens etc can charge high prices for PLCs with a functionality that could probably be implemented on a $20 Arduino. Someone like a train operator does not want the risk of a $multi-million train out of action just to save $100. But providing meaningful after sales service is expensive and has almost nothing to do with programming.
...R
Anything you write is copyrighted, automatically.
Widespread publication may be better at protecting your code than registering your copyright.
If you care, when you publish your code, you should include a license specifying how it can be used and who can use it. There are some 'standard' licenses that are probably applicable.
If you want "no commercial use", your publication should say that in clear text. Otherwise, likewise. For example, some of the Arduino core is licensed under the LGPL license, which is a big ambiguous WRT embedded systems. But Arduino has said that their intent is to allow commercial, proprietary use, and that helps a lot.
There is also this nice clear statement in one piece of SW I've seen:
Licence
Qfplib-M3 is open source, licensed under version 2 of the GNU GPL. Use at your own risk. Qfplib-M3 is not licensed under the LGPL. Roughly speaking, this means that if you wish to use it in conjunction with non-GPL code you will require an alternative licence: please enquire using the e-mail address on the home page.
If you are lucky, a big company will copy your code (or patented idea) and after 10 years or more in court you get a big payoff! And if you have a solid legal case you don't have to pay the lawyers in advance, they just take 40% of the settlement at the end.
If you're lucky, a big company will notice your published code, and your license terms, and contact you about buying some rights to use it, despite the "non commercial" status. Big companies tend to be more careful about this than small or mid-sized companies. The Big Company I used to work for had training for the SW developers on when, where, and how they could use OSSW, as well as a legal team standing by to make final determination and/or alternative licenses. Small companies are more likely to be "oh - I found that code on the net and I thought it was OK to use it."
Note that copyrights protect the expression of an idea, not the idea itself. So even if you have well-defined copyrighted code, a would-be thief can take it, re-write it slightly (say, convert from "the Arduino Language" to pure C, C#, Python, or Go) and not be infringing. Protecting an idea needs a patent. Software Patents are "of ambiguous morality", and much more expensive to get.)
Companies hate to pay for patents or copyrights
Nonsense. For example, I know of a small company that has commercially licensed QFPLIB code from the guy whose license I quoted up above. As employee of Big Company, I've licensed commercial software for inclusion in a product for tens of thousands of dollars (per year? I forget.) It's entirely worth it if you're getting code that would have cost you much more than that to write.
(Now, the chances of getting paid much for some random Arduino sketch that you threw on Github are pretty slim. Significant money gets paid for difficult software with excellent documentation, and SUPPORT behind it.)