This is a home alarm system that is not just a fun toy, but is actually usable and useful in everyday life. I built it because I realized that I already had most of the necessary components for it, and because I wanted to see whether I could make it work. This means it is not the cheapest or the simplest solution, so while describing the system and how to build it, I will also mention possible ways to simplify or customize it to your needs.
The system consists of 2 units, both of them inside your home. The main one (sender) detects when the door is open, and the secondary one (receiver) sounds a siren that is loud enough to attract attention. When you leave your home, you touch an RFID card/tag to the reader next to the door to activate the system. When you come home (or someone else opens the door) a wireless signal is sent instantly. The secondary unit receives that signal, and a countdown is started, giving you some time to enter your home and disable the alarm without disturbing the neighbors. When you touch your RFID card/tag to the reader, a second wireless signal is sent that stops the countdown and turns the alarm off. If the second signal is not received (because the person entering your home does not have the correct RFID tag) the siren kicks in and makes a loud noise for about a minute.
A few things worth mentioning before we go into the details: this system covers 1 entrance only, so if you have more than one doors or windows where someone can break in, you will need to modify the setup and the code for that. The siren will only scare burglars away if there are people in the area whose attention it might grab. Therefore this specific setup is best suited for an apartment in the city and not so much for a house in the suburbs or in the middle of the forest.