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On the day before yesterday, I saw the circuit diagram for a Mobile Phone Detector.

Schematic

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 questions 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 5 km. distance away from circuit?

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

JYelton
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user3580208
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    The circuit works by detecting radio frequencies transmitted by the phone. If the phone is switched off, it's not transmitting anything and thus cannot be detected. I'm not sure about modification of the circuit for improved range, so I'll leave this as a comment only. – JYelton May 09 '14 at 06:33
  • A device that can detect a deactivated phone from 5km, if that were possible, would also detect every other electronic device in 5km unless you attached something to that one phone that would respond in a unique way. The recent NSA documents showing their devices listed several tiny "reflector" devices that could be tracked over long distances by beaming RF at them. – John U May 09 '14 at 08:23

1 Answers1

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As mentioned in a comment, if a cell phone is completely switched off (or put into "airplane mode"), then there is no way to detect its presence with this circuit.

The circuit above is designed to work at a very short distance from a phone -- we're talking only a couple of inches -- because it is using an inductor to pickup the signal, rather than an antenna. It is suitable for a teacher to use when walking down a row of students to see if any have their phones on, for example. Because of the close proximity, one student's phone is not going to be confused with another.

Even if an antenna was added, and the circuit appropriately modified, there would be no way to usefully extend the range as you suggested to several km, since it has no way of identifying one phone from another.

So even if you could extend the range of the receiver that far, it would simply tell you if there are any cell phones operating within that distance. Maybe useful in the middle of a desert, but that's about it. With such a device equipped with an antenna in a room with several people, you wouldn't be able to tell one phone from another.

Even with a sophisticated receiver, there is no way to determine the identity of a specific cell phone operating over the air. All of the communications between the cell phone and the nearest cell tower are multiplexed with other users in the area, and it would be impossible to identify if your phone was one of them (at least for us mere mortals, without the secret sauce).

tcrosley
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  • Minor carp: in airplane-mode I imagine there are probably still minor emissions from the processor, screen etc that are theoretically detectable from short range with sensitive equipment. – RedGrittyBrick May 09 '14 at 14:31
  • @RedGrittyBrick -- Agree, I've changed my first line to read "no way to detect its presence with this circuit (since it wouldn't be sensitive enough). – tcrosley May 09 '14 at 15:22