Ask HN: Idea validation for simple home security system?

I'm moving into a new apartment, solo, and as such, I'm worried about break-ins. Putting extra locks on the door can only do so much if it's a weak door.

I was thinking of a little plastic device that you can stick to your wall. The idea is, when it's enabled and your door opens when you're not there, your phone releases an ungodly alarm sound alerting you of an unwarranted entrance.

Would this be possible? I was thinking you can interface with it via Wi-Fi or bluetooth?

Does a product like this already exist?