>> The difference in noise on the 12V line between a stopped engine and
>> running engine in a car is profound and should be rather easy to detect.
>
>I was actually planning on detecting the voltage difference (battery <
>alternator) but I need to detect if the doors close even while the
>engine is running.

One car alarm system I saw described as a project (in Electronics Australia 
IIRC) was to detect the voltage drop on the 12V line when the interior light 
came on. Now this may not be viable if the engine may be running.

I seem to recall the project could detect something like 10-50mV drop in 
voltage due to the draw from the lamp, and used a 555 timer as the detector, 
but I may well be wrong on the latter bit. 

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