sbright33:
I was hoping to use the instructable link above with only a single resistor? Maybe 2ft away without Op amp?
I understand the power output of a cell phone varies with distance to the tower.
That method is just relying on the very high input impedance of a 'floating' analog input pin and would be better called a EMI (ElectroMagnetic Interference) detector. It will respond to all radiated electrical 'noise' above a certain amplitude, with common 60Hz AC house power wiring probably being the biggest detected signal. However that doesn't mean it won't respond to cell phone RF output, and is worth a little experimentation if for no reason then it's so cheap.
Lefty