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.

Step 1: What You Will Need

  • 2x arduino boards: I used 2 Adafruit Pro Trinket 5V boards with the ATmega328p chip, but any arduino board should work as long as it has enough pins to control everything else.
  • Some source of electricity for both arduinos. For me a USB to wall plug cable is good enough because power failures don’t happen often here. You can add a battery either as a primary or secondary power source if needed (depending on the arduino boards you have).
  • RFID reader (arduino compatible): in my case it’s the Adafruit PN532 NFC/RFID Shield. This is to identify authorized people, so you can replace it with a fingerprint reader, a keypad to enter a numeric code, or anything else you can dream up.
  • A couple of RFID cards/tags that are compatible with the RFID reader (typically 13.56 MHz). In the code I have 2 valid tags: one for me and one for a guest. You can add more for your spouse/flatmate/housemaid, or if you’d like to hide one at home just in case you lose yours.
  • Wireless communication (at least 1 sender and 1 receiver): I used the XBee Module - Series 1 – 1mW (with USB adapter) because I already had those, but you can probably make it cheaper by getting something else doing wireless point-to-point communication. If you want to extend the network, the XBEE module is capable of handling more than 2 endpoints, but the arduino library I used doesn’t support that, so you will need one that does.
  • A display to tell you whether the system is on or off, and to indicate how much time you have before the siren starts making noise. This piece is completely optional, or can be replaced by an LED. But keep in mind the psychological effect when someone opens your door and the first thing they see is a red and black screen saying “alarm in 10…9…8…”. I used a 2.8" TFT Touch Shield for Arduino from Adafruit, because I already had it. It also functions as a touch screen so you could use it to turn the system on, or to enter a PIN code to turn the alarm off. (I decided to use the RFID reader for that.) If you want to use a screen, the simplest one should do the trick, just make sure it’s arduino compatible.
  • Some sensor that will create a signal when the door is open. I used a reed switch (normally open) and a magnet, but it could be a motion or proximity sensor. If you decide to put something on the door and/or the frame, you can use double-sided tape.
  • A piezo buzzer or something to make enough noise. The one I have produces around 95dB which should be enough to scare the crap out of someone from a few meters away, and to start annoying the neighbors after more than just a couple of seconds.
  • You’ll also need a decent amount of wire, and you may or may not need to do some soldering (beginner level) or a breadboard depending on the components you choose.
<p>That's neat :) Well done!</p>

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