Power Saver

I want to build a circuit that would turn off the lights and fans if there is NO ONE in the room...
what are the basic ideas that i could employ to detect the presence or absence of a person in the room

Occupation detectors of several kinds are available at the electrical store, they work on several infrared, active, passive, zonal principles. Get one, dismantle it, reverse engineer to a schematic and put it together again. You will have all the right parts on hand for an unbeatable price.

I'd say use motion OR'ed with sound [filtered for traffic noise] detection.