> On Jun 2, 2008, at 8:40 PM, Jinx wrote:
>
>> The IR transmitters sound interesting, and would lie somewhere
>> between 1 and 2 above, but might be tricky to place so that they
>> aren't also blocked by the passengers or cargo
>
> If an IR transmitter pings into the car space with enough power
> it may not matter that something is in the way. And it's unlikely
> that something would be in the way as you get closer to the hinge.
> People would generally be sitting way back by the door latch,
> perhaps a good 60cm from the hinge
>
> We know how sensitive a VCR receiver is if the transmitter is in
> the room, but point the transmittion out the door and it's lost
> -----------------
> Is there enough of a round-trip resistance difference in a car
> body when a door is open ?


There might be a difference in how sound is transmitted through the  
body.
cc
> -----------------
> I wonder if ultra-sound might work. If you flooded the car space
> with it, would opening a door cause enough of a 'leak' to detect,
> over and above the movement of people within ?
> -----------------
> Detecting an interior ceiling light would be simple, but getting the
> wiring tidy might not be easy for the mechanically-challenged
> -----------------
> Most cars now would have a door indicator light on the dash ?
> Perhaps tap into its supply or a photo-detector on the dash (not
> very aesthetic)



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