Proximity sensor?

[Paul Perry]

At 09:53 PM 29/09/99 -0400, Andrew Schrock  wrote:
 I was thinking some sort of IR
>switch, but I've always had a bit of trouble with those being reliable. I
>need it to turn the sound on when somebody walks in front of it and go off
>when they walk away. Seems fairly simple, but it has to be rock-solid. Oh,
>did I mention cheap? :) 
>
those burg alarm things wd do, you can sheild them to narrow the
field of view.

paul perry melbourne australia 
PIR detector is the keyword. They are hackable I believe.