Troubleshooting a Cell Phone Detector

Discussion in 'The Projects Forum' started by fadelo, Apr 8, 2012.

  1. fadelo

    Thread Starter New Member

    Apr 12, 2011
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  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    The circuit in your link doesn't make much sense to me. The CA3130 is spec'd at 15MHz which is usually the -3dB roll off point. This is many orders below the cell phone band. The output of the chip feeds directly into the base of an NPN with nothing in the Emitter but a LED to limit excessive base current. The capacitor values at the input seem absurdly large at cell frequencies too! These are just a few objectionable things. For sure they're more.

    Regarding your tests... It doesn't require much to detect a cell phone dialing out or when it echos from an incoming call. Just about any AM radio will pickup the packet envelope (which is low freq) when right next to it. In fact my radio starts pop-pop-popping about 2 to 3 seconds before my phone starts ringing. I've always assumed that what I'm hearing is my phone sending an AK back to the cell tower, confirming receipt of the call.
     
    Last edited: Apr 8, 2012
  3. wayneh

    Expert

    Sep 9, 2010
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    You're turning your meter into an antenna, one that works better than the designed one. The tiny impedance of the meter is not likely to be changing your circuit, but you could test that by putting a 1MΩ or 10MΩ resistor where you are placing your leads. But again, my hunch is the antenna effect.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    May I second that? ;)

    Edit: To this day I still say that something went terribly wrong when we advanced from VTVMs to DVMs. The young design engineers of the period must have looked at the old VTVM and said "What the hell is this 1MΩ resistor doing in this probe? :rolleyes: That's why a keep a second probe with a 100KΩ inside. It's high enough to provide test lead isolation without causing a substantial drop in voltage reading. ;)
     
    Last edited: Apr 9, 2012
  5. fadelo

    Thread Starter New Member

    Apr 12, 2011
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    You have a point and I didn't consider to build it until I saw a video about it :
    http://electroschematics.com/1035/mobile-bug-detector-sniffer/

    you are totally right I put a 1.5MΩ in series with one lead and the circuit is wont work again.

    So how to solve this problem? How could I test the OP-AMP in the circuit to figure if it is not defective?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    I think the cell phone is so close to the circuit its high power causes the inputs of the opamp to be severely overloaded then it gives an output.
     
  7. fadelo

    Thread Starter New Member

    Apr 12, 2011
    22
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    I canceled (C1) and it works just perfect reconsisdring the distance between the cell phone and the circuit...

    Thnaks for you all.

    Regards
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    FYI. That circuit shows a "Buzzer" connected to the output of the 555. If it's actually a buzzer, which is electromechanical, and not a Piezo, then you need a protection diode across it before it pops your 555.
     
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