Well, what you need might be PIR sensor. Try this link:
www.glolab.com

This is a passive IR sensor. It detects the change of thermal IR
radiated by our body (around 10um). I've ever found the KDS9 model as
cheap as $1. While PIR 325 with better selectivity cost about $30.

On 7/2/09, 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?
>