how to detect a mobile using a mobile detector even when the mobile is in idle state(ie. when there is no call or message)??

http://electroschematics.com/wp-content/uploads/2009/01/cellphone-detector-550x257.jpg

using this circuit we are able to detect when there is a call or message but not when the mobile is in idle state.

 

what should be the minimum voltage at base for the transistor BC548(npn) to turn on in this circuit?

 

You can't detect an "idle" cellphone as they switch to extremely low power mode and give off no signal you can detect.

If you get close enough (a few inches) you could detect the cellphone with a metal detector.

 

Idle cellphones do periodically send out a "Hello" signal so the cell system knows where it is, but I don't know the period between transmissions. Also the signal is likely to be rather short also making it difficult to detect.

 

Yeah that's many minutes between "hellos" and the signal lasts for less than a second. You can hear it on nearby audio amps and radios.

 

what should be the minimum voltage at base for the transistor BC548(npn) to turn on in this circuit?

The voltage depends on the LEDYou select. For a normal red indicator LED, the bc548 will need 2.6v. Now, that doesn't mean this circuit will work - see above comments.