irishluck:
I think I got it. It senses IR changes in its path.
I just did a search as far as how far it can detect. Read somewhere that it can sense up to 10m?
Does that sound correct?
Yes, but if you tilt the sensor to face the inside of the car then you can limit undesired triggering. The lens is often fixed to a certain sweep and distance and you can usually set the sensitivity through a potentiometer on the board. you can hack the fresnel lens as well by putting some electric masking tapes to avoid "shooting too large" and only bring in radiation that are happening in a limited range.
--> Experimentation is key!