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How do i mod a design? replacing the door chime in motion activated camera?

I've hit a snag building the KipKay motion activated camera.
I refuse to pay 30$ for a door bell chime, the same price as the DVR i bought for it.
Let's find a better solution. So far i could pick up a harborfrieght motion deviceand mod it; or i could use the 10$pir modulefrom radioshack.
A 15$ motion module with a trigger is the goal, lets see if we can do it

edit: could i use the pir as a replacement for the std switch?.......i think so but i need someone who knows for sure, if you need a diagram for the PIR