Copper vs. Zinc Pennies

Discussion in 'The Projects Forum' started by tracecom, Apr 3, 2013.

  1. tracecom

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    I am back on the idea of an electronic way to differentiate copper pennies from zinc pennies with a copper plating. Hey, I know it's a waste of time, but I am intrigued by it.

    My basic scheme is to position some strong magnets on one side of a non-ferrous channel and a coil on the other, and cause the pennies to move between the magnets and the coil so as to generate a voltage in the coil. Today, I messed around with rare earth magnets stacked up on my desk, and placed a penny on top of the stack. Then, I took a 25 foot spool of 22 AWG wire and connected the ends to my DVM. Then, I moved the spool across the top of the penny as quickly and as closely as I could.

    I saw a voltage on the order of a few millivolts, but I couldn't really concentrate on the DVM while I was waving the coil. I tried it with a copper penny, a zinc penny, and without a penny and saw a voltage in all three cases, but of course I had no way to say which produced the higher voltage.

    If I could determine that there was a repeatable difference in the induced voltage using a copper penny versus a zinc penny, I would be on my way.

    What say the brain trust here? Am I "penny wise and pound foolish," or does this idea have some merit? What would be the optimum configuration for my coil?

    Thanks.
     
  2. Markd77

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    Sep 7, 2009
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    How about attaching two pennies, one of each type, opposite to each other, to a wooden disc.
    You can then spin the disc in between the coil and magnet and see what happens on a scope.
     
  3. gerty

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  4. tracecom

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  5. Ron H

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    I can tell you that some hobbyist metal detectors can distinguish between the two. I have a Minelab Explorer SE, my buddy has a Minelab E-Trac, and both can easily discriminate between them. My Fisher Gold Bug Pro can also do it.
    Not much help, I know. I just wanted to let you know that it can be done electromagnetically.
     
  6. tracecom

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    Actually, that does help. I have been reading since making my post about eddy currents, and am beginning to understand. There are several contraptions that use eddy currents to differentiate between copper and zinc pennies, but they all seem to use physical motion. I want to do it by measuring the current induced in the coins, or if not measuring it, at least using it to tell the copper from the zinc. I guess that's the way metal detectors work: the eddy currents are induced, and then picked up and used to alter the frequency of an oscillator?
     
  7. Ron H

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    As best I can tell, my Minelab drives a sequence of pulses of various widths into the transmit coil, and apparently does something like an FFT on the return signal, possibly analyzing amplitude and phase, if that's possible with an FFT.:confused:
    Pulse induction (PI) detectors measure the decay characteristics of the induced eddy currents. The only discrimination capabilities i have heard of on the market are ferrous vs nonferrous, but there are almost certainly PI discrimination techniques that I know nothing about.
     
  8. gerty

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    MrChips and Ron H like this.
  9. tracecom

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    Now, you've done it! Another crazy idea for me to think about. :)
     
  10. gerty

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    I have a lens just like in the video:cool:
     
  11. BMorse

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    I have had one laying around that I took off of a 50" projection TV, now I know what to do with it!:D
     
  12. Ron H

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    It could be dangerous just carrying one of those things around on a sunny day.:eek:
     
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  13. tracecom

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    I was thinking that same thing. If I had it in a frame like the guy in the video, I would have a cover or something to keep the sun off it when I was carrying it.

    I can just see the headline: "Man Immolates Self with TV Screen."
     
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  14. tracecom

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    Seriously...sort of. One of those could be mounted with a sun tracker on a big lazy Susan, along with a steam engine and a genhead, and produce some serious electricity.
     
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  15. BMorse

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    Or some kind of solar cooker, summers coming, it would be one heck of a way to cook some burgers and hotdogs :D
     
  16. Markd77

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  17. tracecom

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    That's a cool...er... hot video. I also saw one of a Fresnel lens heating water for a steam engine. So, now, I am really thinking about generating electricity. The idea is imminently realizable; I knew I should have learned to weld.
     
  18. gerty

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    Some of my students want to focus the lens on our little solar panel trainer, said it would "turbocharge it" :eek::eek:

    Sorry about hijacking your thread.
     
  19. tracecom

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    No problem. Maybe thinking about Fresnel lenses is better than sorting pennies. :)
     
  20. Duane P Wetick

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    Just a thought about non-ferrous alloys and electromagnetic fields. An AC current applied to an air-core coil would show a certain current and phase. Putting a ferrous slug thru the coil would increase the inductance, reduce the current and shift the phase. A copper penny would increase the current due to eddy current effects and shift the phase. A zinc (penny) would also increase the current, but possibly slightly less than the copper penny and shift the phase as well. Both current amplitude and phase could be used to discriminate between ferrous and non-ferrous alloys. This is the principle of operation of many modern metal detectors.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
     
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