Hi,
First of all thanks to charls for the nice link ,and now Mark it is
not our duty to mention the sensor that you want to use in your project
you never stated your goal what you want to do?? I think you want to detect
infrared source in a room that's why you are using PIR sensor otherwise
ultrasonic sensor is a perfect choice to detect motion be ready to bare
all the erratic responces of PIR sensor I hav'nt seen any occupancy sensor
working well.
I assume you already know its principle now
be cautious as the stability of PSP parameter gainst temp change is
of much importance because a change in the slope of coordinate curves
or its zero shift would cause substantial error as the foccused image
moves across the sensor's sensitive layer while calibrating also consider
when the image size foccused on the lense that is whether the source is near
or far this is also a reason of erratic switching beside electromagnetic
radiation,internal noise,humidity and ofcourse as I have said ZERO
drift is the main cause.I have noticed these sensors are such sensitive
that even the movement of your curtains keep them switching on so Mark
think twice before you calibrate,I am not scaring you do give a thought
to thermal changes in your room ?it is very very sensitive passive IR.



hope this helps
sanjeev