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Mobile Phone Detector

Mike McKean
Hi,

If anyone can help me, or explain this, I'd appreciate it. I normally like to understand what I am doing, but on this occasion, I just don't. But I found several different Youtuibes and Diagrams that all used trhe same I.C. But it just does not make sense to me.


I am building a mobile phone detector. I have ordered some parts, based on some web-based reserach I have done. So too late to change. But the parts cost pennies so it's just my time I am potentially wasting.


If mobile phones operate at about 0.8 GHZ to 3.4 GHZ, then I'd expect to build soemthing with a receiver that is a tuned circuit, or several different tuned circuits, but operating at those frequencies. But the I.C. everyone seems to be using is the LM386, which is an audio amp?  I just don't understand it?


I want to detect a mobile phone in "idle", i.e. even when it is not actively mbeing used.


I attached my roiugh working notes which include a couple jpeg diagrams that I am using as references.


Any help appreciated, I am building it anyway. But if anyone has a better solution, or any guidance, I'd be pleased to be educated. Thanks, Mike
  • 0
    HI,


    You cannot  detect a phone in Idle mode unless it happens  to be doing a LA update. This is the only time it will transmit apart from responding to a page.  Anything else is not legal and not acceptablw.
  • 0
    Hi Gerard, what is an LA Update?

    Thanks, Mike
  • Former Community Member
    0 Former Community Member
    When idle, a mobile phone does not transmit anything so you wouldn't be able to detect it. As Gerard said, the phone would only send "on its own" a Location Area (LA), or a Routing Area (RA) update message when either moving between areas, or periodically, based on timers defined by the network. If you are a network operator you can request a mobile to perform LA / RA update - something used when looking for missing persons. 


    Leaving all these behind, I am wondering what exactly you expect to detect. In an environment with many phones active ( i.e. outside a Faraway cage and in the real world ) there will be transmissions by several devices, occupying all, or most of the mobile spectrum. You wouldn't be able to detect a transmission from a particular mobile, unless you had a way to filter out all other transmissions ( maybe with the use of a directional antenna? Still you could have multiple transmissions crib the same direction ). It would therefore be very difficult to detect the transmission from a specific mobile, or to even detect when a new mobile enters the area. You can look for " RF fingerprinting " which tries to identify a terminal through the unique imperfections of its RF circuitry. Very challenging area.... 


    Good luck with your research!
  • 0
    Hi Mike - the 2nd circuit you attached appears to be a broadband RF diode-detector, followed by an audio amp IC which is used to amplify the tiny detected signal to be large enough to light an LED.

    Such diode detectors can be very broadband if they are designed well.

    They are not very sensitive and will only detect fairly large signals.  Also, unless you include some filtering, it will detect all signals picked up by the antenna.

    If you actually wanted to detect (or even block) a phone, a cunning way to do it is to trick it into connecting to a local fake base station - advanced stuff!
  • Former Community Member
    0 Former Community Member
    Hi,


    I think you have the wrong Michael. I would not design such a crude circuit!  The only circuit I have ever designed along those lines was a detector for radar in the 5GHz band, to test the operation of WiFi routers to avoid the radar.


    Best wishes


    Michael Fitch

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