RF Signal/Mobile Phone Detector

Ello people, am looking for a nice circuit that can detect UK mobile Phones. The device must detect the RF signal and produce a light signal as the output to indicate that a mobile phone is being used. ANyone got a circuit!

Oh by the way iam not sure how the antennas will work to detect the mobile! :?

i think this is JUST what ur looking 4. its directional 2, so u can hunt someone down