I have written code at home, on my own time and using my own knowledge and equipment, while under no contract or NDA. I want to make this code open source so that I can use it in software I write for an employer, without denying myself the right to use it at home or elsewhere later.

I'm not sure if saying it is in the "public domain" would fit this purpose, or if I need to find an open source license. I want anyone to be able to use the code in closed source proprietary software with zero requirements for including a license with the source or binary. And I want to minimize the risk of anyone being sued for using it. (I'm aware that one can never be 100% safe from being sued.)

Is there an open source license that fits this purpose? To what extent is what I want to do even possible?

I wouldn't mind putting the license in comments in the code files themselves, but that obviously doesn't go with the binary.