Need help with IR alarm circuit

I'm building am Infrared trip alarm, so that when the beam is cut it sounds the alarm, I found an image that would be perfect but the alarm sounds when it detects IR not when it no longer detects IR, how can I change it to do what I want?

The instructions in Chemelec's project say to connect the beeper from pin 3 of the 555 to the positive supply. Then the beeper will beep when the beam is broken but will stop beeping if the beam is shining on the photo-transistor again.

You can do the same thing with a transistor replacing the 555.