Even though the alarm system is pretty old, its design is hacker friendly. The motherboard has a 12V power output that can be used for future extension cards. This is the power output I used. When something is detected, the siren is triggered by an output and the voltage drops from 12V to 0V.This is very convenient for my project !
The system must be able to notify me in any conditions, for that reason I wanted three warning types:
-Email : Even if I don't have my phone I can get notifications on my laptop
-SMS : The most convenient way to notify someone
-Call : At night my laptop and my smartphone are turned off, the only communication available is my fixed-line phone
Interfaces and Components
The first type of notification is easily doable with Python on the Raspberry pi. The two others will be handled by a GSM modem.
For the modem I used a SIM900 : a popular component with a great online community support.
I needed two power supplies, 5V for the Raspberry pi and 4V for the GSM module. I used two TI PTN78000W series because they have a high efficiency and great output current.
The last and more important interface is the siren trigger. I used optocouplers to step down the voltage to 3.3v and protect the Raspberry pi.