I need to detect an operating cellular(mobile)phone, not to spy on call but just to detect its presence?

Question

I need a tip , a simple circuit that i can assemble. or a web site that contains such circuits. Required detection distance is 1.5 meters or less.

Answer 1

If it's just a signal strength meter you want that isn't particularly frequency conscious then http://www.qsl.net/n9zia/wireless/pics/fs_meter.png
should do it. Instead of the illustrated 2.4GHz aerial, you need one for 900MHz and one for 1800MHz depending on which phone band you want to detect.

Answer 2

Firstly you will have to decide whether you want to detect a CDMA phone or GSM phone. For GSM case signal is evcrypted.GSM system uses several mecanism for encryption like A5/0,A5/1,A5/2 etc.Of these A5/1 is toughest to break.But if you search internet , you will find a lot of articles about intercepting GSM.
In case of CDMA , it is very very difficult to intercept.Because it uses code divition multiplexing,Direct Sequence Spread Spectrum techniques which are very strong techniques.Also the spectrum of CDMA Signal is more like a noise signal so it is very hard to distinguish it from noise and other signals.

Answer 3

From what I understand you want detect if a PARTICULAR phone is present This would not be possible as the data transmissions would be encrypted. If you just want to see if ANY phone is present just find out the operators frequency range and use a "wideband amplifier" for that range and correct antenna