Hi Matías, I am just seeing an arduino alarm set for a couple of weeks (I am still analyzing everything) but as I was wondering about using some keyboards (which seem more robust to me than the keypads sold with arduino) and existing IR sensors (more prepared to use them in an installation) ... I pass you a link of a video that I found where it shows the keyboard connection and this one as connect IR sensors maybe they can help you a little more, if at some point I decide to put together and try something I warn you, regards