
I took the idea of this project on my last holidays. We were out of home for twelve days, and I set some automatic rotary switches to pretend life inside the house while we were at our vacation resort.
This, of course, is a rough try, but I think that is better than nothing.
The idea was to attach a pyroelectric detector (PIR) inside the door’s ‘surveying hole’, so when motion outside is detected it were pretended a more realistic events in the house. That is, noises, turning on-off some lights…, that’s all.
The second part of the idea was to have instant warn by mobile phone call if “too much” activity was detected by the PIR detector.
In that case, I could ask my neighbour to take a look in order to know the details of such suspicious situation.
Many mobile phones have AT-Hayes command set integrated. This means that the phone can be mastered as though it were a modem. It can be issued calls, SMSs, and other actions by sending AT commands through its serial port (when available ...).
Since I had to deal with the extended AT commands I went through all the documentation I could find on the web with regard to AT-Hayes and ‘Short Message Service’ (SMS). After a few days I became an expert : -).

The circuit is built around a PIC16F876, SMD version.
The operating method is the next:
Whenever the PIR detector detects any activity a phone call is issued to the pre-programmed telephone number.
When I receive the phone call from my 'alarm system', I hang the call so there is no cost for each of these warning calls.
If the battery of the alarm has been drained too much, a SMS with the text "LOW VOLTAGE" is sent to that pre-programmed phone number.
There is also a tilting sensor on the device for anti-tamping purposes. So, if the board is taken by any stranger the alarm sends a "PANIC!!" SMS to alert the owner.
After this , the system auto-protects itself by automatically switching off the mobile phone, and blocking the alarm.