Hi Jochemham,I'd start by reading the tutorials on this site and prototyping a few things. First, learn to understand switches, which should be fairly easy.Second, get a buzzer and learn to turn it on and off via a GPIO pin and a transistor,Third, download some code for the IR sensor and get that to work the way you want.Then start to put them together in an orderly manner to build-up your final system.I made a similar alarm system with a PING))) sensor to let me know when my dog needed to go outside.There are a lot of tutorials around on programming and simple electronics. You can't beat learning by doing! It sounds like an interesting project.Try some things and come back when you get stumped. There is a lot of help here.Pat.