Parts and tools needed:
Something to cut the pcb
1x tip31 transistor
1x Airwick motion sensor
1x Wireless doorbell
Step 1: Digitized sensor input
Tnx Doug Paradis, this step gave me some trouble.
I only changed a part of his method, I just used some solder and no drilling.
Step 2: The Idea
EDIT: I recommend you add a power switch between the center tip31pin and the negative circuit contact, the circuit loves to eat batteries!
The tip31 can be replaced with 2n3055 :)