This alarm system senses motion and then alerts you by phone. [Oscar] had an old external modem sitting around and, with some wise hardware choices, he came up with a simple circuit to use it. First up is the PIC 16F628A chosen because it doesn’t require an external crystal. This connects with the modem via a DS275 RS232 transceiver because it requires no external parts for connection. The final portion of the puzzle is a PIR sensor that triggers a pin interrupt in the sleeping PIC, which then dials your number to alert you. It doesn’t look like anything happens other than your phone ringing, but that’s enough for a simple system. We’re just happy to see how easy it was to use that modem… time to go hunting for one in dreaded junk trunk. Don’t miss the clip after the break.