Hello All

Hope I'm posting this in the right section.
I've got a problem and I can't find a solution ready made so I'm trying to make one myself but could do with some advice... Ok i'll be honest and say I'm completely stuck and need help.

The Problem

I'm trying to create a type of intruder detection system. Nothing too high tech. I've got a laser trip wire over a door way. This is a normally open circuit that pulses every 100ms and triggers if the beam is broken. Standard stuff. But what I want to do now if plug this into a nearby pc using a usb rs232 to rs485 converter. And have the pc detect the trigger (broken beam) and play an audio file.

I've already tried a setup my friend had as part of a DVR (digital video recorder) in other words cctv which had its own input card which the triggers and can play an audio file but as soon as the beam re-engages (someone walked through) the audio stops so it only plays for about a second. Plus this solution would cost quite alot I'm just trying to keep it as simple as possible.

So in short I need a piece of software that can be triggered by the input from the usb rs232 to rs485 convertor and then play the audio file in its entirety (30 secs) then wait for it to be triggered again.

Any help would be much appreciated.

I posted here because I figered I could try and write a program if no-one knew of one that exsisted but I'm not really a programmer so hoping for advice