Project: Wire break detector

Discussion in 'The Completed Projects Collection' started by wayneh, Nov 9, 2011.

  1. wayneh

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    Sep 9, 2010
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    Here’s a project I finished to help a friend find a break in her buried-wire dog fence. It employs a DIY AM transmitter designed specifically for this project, and a standard AM radio.

    Background: An invisible pet fence is usually a loop of wire, barely underground, acting as an antenna. (There are other types, including a portable transmitter that causes the dog to be zapped when he goes out of range.) The transmitter unit is wired directly to the antenna and placed out of the way, in the garage for instance. The pet wears a receiver on its collar and receives a jolt when it gets too close to the perimeter wire. The signal is probably coded - you wouldn’t want just any broadcast at the right frequency to zap the pet. Many commercial units reportedly broadcasting at 600kHz, others (particularly PetSafe) reportedly at 10.65kHz.

    If you have a 600kHz system, there's a popular method for finding a break in the wire loop that involves placing a 100µH RF choke onto the transmitter, in parallel with the broken loop. The transmitter detects a lack of continuity when the perimeter wire loop is broken and sounds an alarm. The low DC resistance of the choke allows the system to continue transmitting without the alarm. But the inductance of the choke presents a high impedance to the RF signal, so the RF signal is sent out over the antenna pieces. You then use a portable AM radio tuned to ~600kHz to scan along your perimeter, listening for the lack of a throbbing signal, indicating the location of the break. Many folks have had luck with this, so I'm sure it works at least for some systems. Note that some write-ups suggest you additionally wrap the ends of the broken loop wire around the leads of the choke before reconnecting to the transmitter, instead of just putting them in parallel. Wrapping the boundary wire leads around the choke leads provides a crude isolation transformer. Pulses through the leads of the choke are picked up by the wrapped wire. Perhaps this increases the signal you would get without wrapping?

    I found an old solar light that had a 100µH inductor on it (brown, black, brown, silver, measured resistance at ~3Ω, so it's not a resistor). The technique didn’t work for me though, as my friend’s system is PetSafe, and doesn’t broadcast in the AM band. The PetSafe solution is an expensive (>$50) accessory consisting of a transmitter that generates a two-tone signal in the AM range. It comes with a receiver.

    Strategy: So I started thinking about making either a transmitter or a receiver that would allow me to find the break. I soon realized it would be easier to build a transmitter and use a standard AM radio as my break finder.
    1) This is simpler than building a receiver for 10.65 kHz, the native PetSafe dog fence frequency. (Which may vary between models.)
    2) I can test my transmitter with a standard AM radio at 1000 kHz, and use the same AM radio to find a break in the wire. If I build a receiver for 10.65 kHz, I cannot test it except with the original transmitter, which I cannot assume is working correctly.
    3) This solution will work for every buried-wire system since it uses no part of the original system, except the perimeter wire itself.
    4) The transmitter strategy is used in the $50 commercial device.

    Schematic: The circuit I started with is described here and http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html. (The second link has better pictures but muddies the purpose of the transformer.)

    [​IMG]

    Note how the high impedance side of the transformer limits power to the oscillator, allowing the pulsing of the audio signal to amplitude-modulate the 1MHz carrier provided by the crystal oscillator.

    Since I didn’t have any suitable transformer on hand, I experimented to see if I could replace the audio transformer with a ~1k resistor in series with power to the oscillator, and a capacitor to couple and isolate the audio signal. My concern was not losing fidelity, since I just want to broadcast a continuous tone, say 1 kHz. But I wanted to be sure to not overload my tone source, my iPod’s headphone output, which I think is 32Ω impedance. My iPod survived all this, so I guess it’s safe as shown. Anyway, it worked fine to omit the transformer.

    I also wanted to be able to use a wide range of power supplies, so I added a voltage regulator. And, I wanted to put out TWO tones onto the opposite sides of the broken wire. A break in the wire will be detected as a pitch change when you scan over the break, from one tone to the other. This required building two identical AM transmitters. I routed the two stereo channels to the two transmitters.

    My final schematic:

    [​IMG]


    The build:
    [​IMG]

    [​IMG]

    [​IMG]

    To use the device, I made a 6-second “song” containing one tone on the left channel and another on the right. Sorry, but I can’t recall what software I used or what frequencies I used. The Mac application may have been Audio Toolbox. The output was something like a 440Hz sine wave on the left and 1,320Hz on the right. [UPDATE: I since recalled that I used NCH Software's program "ToneGen" - free trial - to generate the sound file, using 440 and 1,760 Hz on the left and right channels respectively.] I recall thinking afterwards that I should have used tones that are NOT in tune with each other to make it easier to tell them apart. I set my iPod to replay the tone song repeatedly and tuned in my AM receiver. (Mine has digital tuning.)

    Conclusion: It worked! What more can I say? At the workbench, the transmitter could be detected at least about 10 feet away. Detection distance increased with antenna length. In the field with the buried perimeter wire, the signal was easily detected a foot or two above the ground, by sweeping the receiver back and forth perpendicularly to the wire. I found and repaired the break in minutes.
     
    Last edited: Nov 28, 2011
  2. bertus

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    Apr 5, 2008
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    Hello,

    That is a nice little tool.
    You will find the audio signals multiple times on the radio, on the multiples of the x-tal frequency.
    Any AM reciever or SW reciever can be used.
    For generation of the audio tones file, you could use the open source program Audacity.
    http://audacity.sourceforge.net/

    Bertus
     
  3. studiot

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    More folks should publish good projects like this.

    You deserve a chocolate biscuit!

    :D
     
  4. wayneh

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    Mmmm.... chocolate.

    If only you could squeeze one thru the internet to me! :D
     
  5. thatoneguy

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    That is a most clever solution merging new tech with 'old tech' (iPod + xtal/inductor AM transmitter).

    The sparkle of true insight shows through where you split channels to locate the fault precisely.

    A Fox and Hound may have done it (telco equp), but it needs 2 wires, earth and conductor, and you wouldn't be able to get nearly as close to the break as you did, and at such a distance above the ground.

    Your solution even beats out a TDR that gives wire length, as calculating the length and wandering about calculating would have been a pain.

    Overall, I give it 4 stars out of 5. You'd have 5/5 if you would have not use mac and ipod, open source linux tools and a standard mp3 player would have made it 5/5. :p
     
  6. Wendy

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    Old eyes adjustments again.

    I also resized some of the images (with Paint) and put the new images here, the old images are on your thread untouched.

    Hope you don't mind. If you do I'll knock it off. I've never seen the attach /attach option before.
    .
     
  7. wayneh

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    No problem, Bill, I appreciate it. Except that now I have no reason to learn this trick myself! You're an enabler. (Just kidding) ;)
     
  8. wayneh

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    If you dinged me for using a Mac, you'll really clobber me for this one: The two-tone idea is employed by the commercial PetSafe device. I just reverse engineered a patented solution. :(

    Like you, I thought it sounded like a darn good idea.
     
  9. Wendy

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    I think I'll write a sticky for the Suggestions and Feedback forum how to post pictures. I need to learn attach /attach, as I've never seen it before though.
     
  10. bertus

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    Hello,

    Did you ever look in the FAQ?

    Bertus
     
  11. thatoneguy

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    No, that shows that you did a bit of research (unless you just asked on a different forum. :eek:) so it balances out.

    I do have one question about the design: You put it in an Altoids tin, but still mounted a heatsink to the 7805, why not just mount the 7805 to the tin itself? I doubt anything would change with the functioning of it, but I thought I'd ask, since it is a $2-$4 item if you count the isolation mounting kit, mica spacer, and thermal grease. Though I don't know if you use any of that stuff other than the actual heatsink.

    Still, excellent solution. I wouldn't be able to use it since I just realized, I no longer own an AM radio that runs on batteries.
     
  12. Wendy

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    I collect them, and FM recievers, specifically for the odd electronic project. Cheaper and better than building them from scratch. Yard sales are also a good resource for them.
     
  13. wayneh

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    I found a hi-rez image online, read the patent number off it and pulled up the patent. I worked in R&D for years, and old skills come in handy.
    My in-car iPod chargers have the 7805 screwed right to the tin. That works fine although the tin does get hot to the touch - almost to dangerous temps. But current is much higher in those. I can't say I really thought about it one way or the other for this project, though. I think the 7805 had been pulled from something and already had the sink attached. I absolutely did not buy anything and would definitely not have used a sink if I had to buy it. The power consumption of this thing is no where near enough to warrant it.
     
  14. Wendy

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  15. thatoneguy

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    Whoah, I am 100% confused now! Thanks for starting off my day with a time warp. :)
     
  16. thatoneguy

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    I think patents are by far the most under-utilized resource people use. They've usually got the answers staring you in the face.

    The problem is a free patent service on the web w/images now.
    Patent storm.us used to rock,free PDF of any patent, now they want $3 ea. The other sites I've seen only show random images from the PDF to get you to buy it. If you can get a copy for free if willing to wait, I'd think there would be more full patent archives out there.
     
  17. wayneh

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    We're off-topic again but here is where I go for full pdfs of patents. You have to jump a hoop or two but it's not bad. Years ago you had to download one page at a time. Some genetic engineering patents are 150 pages long, so that really wasn't too great. The USPTO site is OK too - full pdfs for free - but is limited to U.S. patents.

    And yes, I believe in standing on the shoulders of giants. A lot of posts in these forums would disappear if people would simply read a few pages.
     
  18. TimberGetter

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    Oct 10, 2011
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    I am referred to this thread in response to a question I raised on the following forum about tracing the route of an underground pvc conduit:

    http://forum.allaboutcircuits.com/showthread.php?p=419370#post419370

    I am planning to push a 50 metre length of steel wire into the 32 mm diameter conduit which is buried at a depth of up to 600 mm. This project would seem to have some prospect of sensing the wire.

    In the event that there is not enough power to penetrate the 600 mm of earth could someone suggest how a simple additional stage could be added to the current project design to achieve that greater power?
     
  19. studiot

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  20. TimberGetter

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    So Studiot, it seems you are not optimistic about the simple radio transmitter working, given that I can push a steel wire up the pvc conduit?
     
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