You'll need to tell the employer before signing any paperwork, for sure, but you don't necessarily need to mention it in your first or second interview unless it comes up. I'm not suggesting you deceive the employer, just that you can take time to evaluate them and let them evaluate you for your skills before providing details on your off-time projects. If they discount you for being passionate enough about development to start your own, non-competitive thing on the side, they're probably not a great fit for you.