From what you wrote I would assume they are talking about one of those IR motion
detectors used on lighting. They don't generate any IR beam of their own. They
receive the IR generated by body heat and focus it on a row of detectors through
a cylindrical Fresnel lens. When the relative levels on the multiple detectors
changes it means something warm has moved. Overall temperature changes affect
all detectors evenly and so are eliminated.

I have not worked with these sensors, only read about them a took a broken one
apart for curiosity.

SherpaDoug

--- In sensorforum@yahoogroups.com, CoooL Khalid <coool_genius@...> wrote:
>
> bros. i m working upon 'automatic room lights ' project, assigned at
university....
> In which we have to switch 'on' or 'off' the lights of a room for a person
entering or leaving the room ,respectively. Now here LDRs are used for sensing
the person, as shown in following link;
>
> http://www.electronic-circuits-diagrams.com/lightsimages/lightsckt4.shtml
>
> there is a suggestion of using an IR module instead of LDRs.... so,now how
would i generate IR signal?
>