How to detect/identify mobile phone if it is switched off?

[sudheerelktronics]

I have recently saw a circuit diagram of Mobile Phone Detector.

It is mentioned that the circuit is helpful to identify the presence of an activated cell phone by detecting these signals.

But, I have two queries about this circuit:

1. If a person steals our mobile phone and he switches it off, then I think we cannot identify that mobile by using this circuit. So, is it possible to modify the circuit in such a way that it could be detected even it is switched off?
   
2. It is mentioned that low range detector. So, how to modify this circuit in such a way that, it should identify the mobile even up to atleast 3 km. distance away from circuit?

Can anybody suggest me the modifications to be done in this circuit?

Thanks,
Sudheer.

 

[brig001]

1, No
2, Yes, well maybe, but it would detect every mobile phone (and possibly Bluetooth devices, wifi and microwave ovens) in a 3km radius. I'm not sure that is what you want.

Brian

 

[DF96]

That circuit will not detect just mobile phones but almost anything which radiates in the VHF region. It won't be very sensitive to mobiles as they work on too high a frequency. It is essentially a 'toy' circuit, not a serious detector.

A question to ponder: how much radio frequency energy does a phone emit when it is switched off?

Another question to ponder: roughly how many transmitting devices are there in a circle with radius 3km in a country with a technological culture?