Don’t hurry to throw old stuff away – they might be interesting and useful for some projects. Check out this alarm system built around modem. Alarm system itself consists of PIC microcontroller and PIR sensor. When someone is detected in the room microcontroller dials a number to alert the user.
Microcontroller communicates with modem via RS232 interface. There is a DS275 chip used instead of well known MAX232 – this way circuit becomes even simpler as there is no need for caps. As there is only one sensor at present probably it would be great to expand project to secure more objects by adding more detectors.