How can I pick up a digital signal on a wire ?

Discussion in 'General Electronics Chat' started by EB255GTX, Sep 9, 2015.

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  1. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Hi all,

    I need to pick up and read into a micro a digital 0 to 1kHz, 10V pk-pk signal on a wire non invasively.

    The signal wire is in a cable that also contains the ground wire for the signal, and nothing else. the cable is not twisted, it is flat 4 core telephone type with 2 cores unused. I can not get into the cable to for e.g. run only one signal wire through a pickup coil.

    I'm happy to amplify/clean up the signal after pickup, but the phyics of picking up SOMETHING to post-process is what has me stumped at the moment!

    The thoughts i have had so far (will try some on the weekend) are:

    - Hall effect sensor
    - Induction - turns of wire on the cable (will the return current on the ground wire stop this from working?)


    Anyone played with something similar or have any guidance?

    thanks!
     
    Last edited: Sep 9, 2015
  2. #12

    Expert

    Nov 30, 2010
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    There are lost of ways to bug a phone line. Start thinking in those terms.
    If you can touch the wire, you can use a capacitor to couple into a very high impedance amplifier.
    If you can't touch the wire, there are inductive pick-ups.
     
  3. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Thanks - the wire is not able to be contacted electrically.

    What i am getting at is, does anyone have suggestions for a circuit to DO the inductive pickup, or should i just start winding turns on to the cable and monitoring with a scope until i see a decent signal (again, weekend job, right now i am in the research phase!)

    What about the hall effect - I am wondering if a HE sensor taped to the wire might pick something up. I wil try all of this on the weekend but this forum is a good place to get info that might save me soem time :)
     
  4. #12

    Expert

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  5. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Thanks - but to be clear - I can not get into the cable which is why i am wondering what i will see with a hall effect sensor - i THINK with the signal and ground wire inside the inductive pickup i will get nothing. Maybe with a hall effect i will get a spike that i can use as detection.

    For claritty - i only need to count the pulses
     
  6. #12

    Expert

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    Quotation from the very first link I gave you:
    "Since the signal is induced magnetically, no direct connection to the original conductor is required."

    Are you saying you can't even go where the wires are already separated and place the coupler around one of the wires? You can't split the cable with a pocket knife? You just want to walk up to any cable and touch the outside of it and get a signal?

    That sounds like a serious problem. By definition, all the current in one wire is equal and opposite to the current in the other wire. The magnetic fields WILL cancel. It might be possible, but I don't know how...yet.
     
    Last edited: Sep 9, 2015
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    If the tap is for digital signals how much induction signal you can easily get depends on the matching of the transmission line for high digital rates as you will normally look at the digital signal edges for the response. There usually is not a return current on the ground wire as it's an electrostatic shield. I don't want to go too far here but if you can alter the physical layout (without cutting or marring the cable) of the cable in some way to cause a localized impedance change this enhances the amount of signal that can be collected.
     
  8. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Yep - unfortunately the cable can not be tampered with in any way, certainly not electrically, and cutting the jacket to get a core over one conductor would i think be frowned upon but might be a last resort.

    The persistent thought i can't get out of my head is that there must be some leakage of the signal from a flat straight cable otherwise why are if there are twisted and shielded cable types available?

    I will do some more research - thanks for the ideas thus far.
     
  9. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    good info thanks, and quite relevant (in a scary way) to your username :)
     
  10. nsaspook

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  11. #12

    Expert

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    This is what I would do:

    Get a sample of the cable with the signal happening inside it and start laying coils or Hall Effect sensors next to it. Watch with an oscilloscope during changes in orientation. If you get nothing, add a differential amplifier with excellent common mode rejection, probably sold as an "Instrumentation Amplifier". This job resembles using two microphones wired backwards to eliminate far field sounds. You are looking for near field differences which are very small. You will quickly arrive at the need for a shielded metal box for the amplifier(s) or they will be swamped by the power line radiation which is all around us.

    That's all I have right now.
    There are smarter people than me on this site.
    Maybe one of them will come along and assist.

    Government agencies and other nut cases probably already have this equipment, but I assume you came here because you don't want to spend $1000 or draw attention from the NSA.
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    Why don't you bribe, black mail or extort someone into steeling the data for you.

    This is an unethical thread and should be deleted.
     
  13. nsaspook

    AAC Fanatic!

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    It depends on what it's being used for. I took courses on locksmithing and picking. If I used it to break into banks that's wrong but breaking into a simple safe at work is not when authorized.
     
  14. Lestraveled

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    May 19, 2014
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    Granted. Where does the OP state that he is doing it for the good of mankind? To me, the obvious purpose of this thread is to glean the knowlege to wiretap data.
     
  15. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Correct. But the data is not secret or illegal for me to obtain, in fact I pay the company to give me the data from this equipment.

    The difference is that i want to see the raw data not the processed data the company gives me (they roll up some of the numbers and so on and i want to do some different statistical analysis prior to that rollup).

    It will not benefit mankind as such but will not hurt anyone or contravene any laws, real or ethical :)
     
  16. Lestraveled

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    You pay to see processed data. You want to see the raw data but, (I assume) the company will not give it to you. Is there an issue of proprietary data or processes?

    You have agreed that the reason you are here is to find out how to wiretap data. This, by its nature is nefarious. If you have a legitimate need for this information, could you be more specific as to why.
     
  17. EB255GTX

    Thread Starter Active Member

    Apr 30, 2011
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    Guilty till proven innocent huh? :)

    I could provide the reasoning, but I won't because it is irrelevant to the solution (i have provided the signal parameters and other technical info needed) and because it would derail the conversation away from the technical solution.

    I have a few suggestions to go on so will get to work testing - thanks to all for the comments!

    p.s. Lestraveled, consider the example of a consumer who purchases one of those energy monitors which have a clip on CT for the incoming fusebox mains feed. Would that not also be "nefarious" by your definition - they would be "wiretapping" since you are not allowed to physically mess with the utility feed, and gaining access to the raw data (current) that the utility processes to give them the information on how much power they have used (in the form of a monthly bill).
    Conclusions can be jumped to from a standing start :)
     
  18. Lestraveled

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    In your example to me, you are confusing intellectual property with a purchased commodity. You bought that current and have every right to monitor it. You did not present a real argument that you had any right to the data you were trying to harvest, so I suspect you didn't, and you pretty much confirmed that in your last post.
     
  19. #12

    Expert

    Nov 30, 2010
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    @Lestraveled
    Report this to the Moderators. They have the power to manage what they think is proper for this site.
     
  20. Lestraveled

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    I did before I posted one word.
     
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