There is a 9v power supply connected to a pair of 9V batteries, these bats are connected in parallel and are powering the BX24 and PIR sensor. The other power supply is an old 5V Samsung cellphone charger that was dismanteled, i use this one to keep the siemens C35 charged, so if the power goes down i have 400mah(200+200) storaged in batteries for the Microcontroller and PIR Sensor and the cellphone battery for its own, this should be enough to keep the alarm online for a few hours.
http://alumni.ipt.pt/~pmad/gsm_alarm.php