How do I identify metal detector Transmit from Receiver Coil ?

Discussion in 'The Projects Forum' started by ToddB72, Jul 19, 2013.

  1. ToddB72

    Thread Starter New Member

    Jul 18, 2013
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    Greetings !

    I am using the Velleman K7102 build-it-yourself Metal Detector kit pictured below to make a pinpointer for use during the recovery of coins, jewelry and other items in the ground while using my metal detector.

    After completing the solder work on the PCB, I will be installing it inside a small plastic electrical box w/plain cover as an enclosure and then attaching a plastic tube with end-caps to the side of the box. The ferrite rod with it's copper wire windings (transmit and receive coils) will be mounted inside the front end of the tube which extends beyond the front of the enclosure box by about 6".

    Looking at the picture below (click to enlarge) notice that the L1 coil has 120 turns and the L2 has 43 turns. I want to install the ferrite rod and coils into the tube so that the signal transmitter coil is at the front end of the tube and the receive coil behind it. My problem is I don't know how to determine which coil is the transmitter.

    Is it possible to identify the Transmitter coil from the Receiver coil merely by the number of windings ? Or does it depend on what circuit components are tied-in with the ends of each coil ? Or is there a test I can do with my VOM after completing the windings on the ferrite rod ?

    Your help with these questions will be appreciated.

    ToddB72
     
    Last edited: Jul 19, 2013
  2. tracecom

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  3. Ron H

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    I found the schematic. I simulated it (making some educated guesses about coil inductance).
    The circuit doesn't really have a transmitter and a receiver. Both coils are involved in the oscillator. When there is no metal object near the ferrite core, the Q of the core is high enough to allow oscillation, which keeps the LED turned off. When a metal object is near the coil, it lowers the Q below the level required for oscillation, and the LED is switched on.

    The bottom line is, it probably doesn't matter which way the coil points.
    If you are not convinced, put the big coil nearest the front. It has higher oscillation current than the smaller one.
    You could easily test the sensitivity by testing both ends, using a coin.
     
    Last edited: Jul 20, 2013
  4. ToddB72

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    Jul 18, 2013
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    tracecom..........Thanks for the helpful suggestion.

    I had phoned Velleman earlier and one of their technical staff "Scott" wasn't able to give me a concrete answer (I believe Ron H's post explains why and offers a course of action for me to try.).

    ToddB72
     
    Last edited: Jul 20, 2013
  5. tracecom

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    I have on of the kits, but not assembled it, so I am interested in your progress. My interest is in using it to differentiate copper pennies from zinc pennies.
     
  6. ToddB72

    Thread Starter New Member

    Jul 18, 2013
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    tracecom.......That's an interesting use of the kit, but you might try the following and save the kit for another project.

    #1..As I'm sure you know, many of the "zincer" pennies that have been exposed to corrosive elements, especially any that were in contact with mineralized ground for any length of time, will be showing signs of pitting, etc.., whereas the coppers will normally have a nice smooth, brown, patina, even when exposed to mineralized ground, so this is one way of initially segregating coins.

    #2.. According to my copy of the 2013 "Red Book" for U.S. Coins, page 122, copper Lincoln pennies were made from 1959 - 1982, weighing 3.11 grams each.

    Sometime during 1982, due to manufacturing cost, the mints switched to copper plated zinc (core: .992 zinc, .008 copper, with a plating of pure copper, total content .975 zinc, .025 copper), weighing 2.5 grams each. I just checked a copper penny dated 1974 plain and one dated 1986 D on my digital pocket scale and these weights are correct.

    BTW, you can buy these little pocket scales very cheap and they are plenty accurate for every-day use on coins, jewelry and other items. I have two different brands, one that I bought at Harbor Freight and another off the web. Suggest you also invest in a test weight for calibrating purposes. Check to make sure the pocket scale you buy has a "Parts Counting Function"; this will allow you to weigh a stack of 10 pennies at a time ! If the total weight is off, you can check the dates to find the pre-'82 or post-'82 odd pennies in the stack and separate those out. Full operating instructions come with each scale.

    So, you could weigh-check your pennies, as one method of separating the alloys and weights.

    #3..Another slightly different method would be to set up a desk light and view your pennies (some might require a magnifying glass.) and separating any that have a date earlier than '82 from the "zincers". Since the mints continued to produce a limited quantity of copper pennies during the first half of 1982, set aside any 1982 pennies for their exact weight later on.

    Hope this helps, but most important, have fun whatever you do !

    ToddB72
     
    Last edited: Jul 20, 2013
  7. Ron H

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    The copper in the pre-'82 pennies is worth more than 1 cent. The zinc ones are worth less than 1 cent. Some people are hoarding the coppers, planning to sell them for their bullion value sometime in the future. To make money at this, you have to have hundreds of thousands of pennies. You need a fast automated sorting process, where they can be differentiated on the fly. Weighing or dating them is probably not fast enough.
     
  8. ToddB72

    Thread Starter New Member

    Jul 18, 2013
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    Ron H........Thanks much ! That helps me to continue with my project. :D

    Sorry you had to go to the work of simulating and guessing the coil inductance. My scanner is down, otherwise I would have included a picture of the schematic from the Velleman manual along with my thread-starter post. (I copied & pasted the picture of the completed metal detector K7102 to my post off of the Velleman website.)

    Best Regards,

    ToddB72
     
  9. ToddB72

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    Jul 18, 2013
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    Ron H,

    I agree with the above. There seems to be hundreds of coin sorting machines of different types and prices on the market for retail.......some are very costly !

    I've only been collecting the pre-'82 pennies for the last few years from pocket change and have (2)18oz plastic peanut butter jars fully packed tight up to the lid seal, which I haven't counted yet, plus $1.52 in a third jar. Regarding these coppers being worth more than a cent each, are the "Coin Star" machines in the stores able to differentiate the heavier coppers and give you a bonus payback.......do you know ? Or would the banks give a premium ?

    Thanks in advance for you reply. ;)

    ToddB72
     
    Last edited: Jul 22, 2013
  10. Ron H

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    I obviously found the schematic. I don't think it specified coil inductances.
     
  11. tracecom

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    I just got interested in sorting pennies a few months ago; I look for them in pocket change. I weigh the 82's to sort them, and just look at the date for the others. In five or six months, I have only found about 150.

    My interest in sorting them electronically is primarily academic. I don't ever see myself sorting them in bulk. I built a small contraption with a microcontroller that had some success, but I want to try the oscillator method. Again, just for the educational value of the experiment.

    As to the banks giving you a premium for coppers, fat chance! If the bank policies don't prevent it, there are some sharp-eyed tellers scarfing up copper pennies and silver coins. My guess is that a lot of banks have machines to sort them. Same thing for the coin machine operators, and the federal government.

    Anyway, thanks for your posts.
     
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