How to redirect from a main website to a mobile version?

[Ashley]

I have a website on my local server and I have a mobile version in a sub-directory (play-it-louder.com/mobile). What I want to do is to put in some sort of code that detects the screen size, so when it gets smaller than, 500px wide perhaps, then it would automatically load up the mobile version,

any help would be great,

thanks in advance!!!