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Induced current in a coil

Discussion in 'General Electronics Discussion' started by seza, Jun 10, 2021 at 1:06 PM.

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  1. seza


    Nov 6, 2015
    If I have a coil of suitable configuration and I pass a magnetized needle
    over it (NOT through it) as close as poss - would that induce a current/voltage in the coil?
    if so could that current/voltage be "amplified" to be used as an input to an Arduino board?

    This is a great forum and I am learning a lot here so a big thank you to those that take time to answer questions.


    Dec 19, 2019
    Use a magnet and magnetic reed switch.
  3. Hunter64


    Nov 20, 2018
    That was not the question!
  4. Minder


    Apr 24, 2015
    Use a analogue input.
  5. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Yes. Faraday's law of magnetic induction applies to any conductor subject to a changing magnetic field. Depending on the strength of the magnetized needle, how fast it moves while passing near the coil, and the number of turns of wire on the coil the resulting output voltage while the needle is moving past the coil could range from microvolts to millivolts. Whatever the amplitude, the output will be a pulse, not a continuous signal... unless the needle moves back and forth parallel with the axis of the coil.

    Sure, but you need to provide a LOT more detail about what you are trying to DO before we can help you. Experimental evidence is helpful too. Perhaps you could connect a digital storage oscilloscope to the coil terminals and record the output as the magnetized needle is moved about the coil. Post some DSO screen shots as well as pictures of the coil and the magnetized needle.

    Thank you, and agreed that this is a great forum. The forum has seen better days, and members such as yourself who joined many years ago may not participate much anymore. A lot of them still "lurk" here once in awhile, and may even chime in with a response if a question interests them.

    There are thousands of narrowly-focused special-interest forums and blogs you could also visit and/or join if you are on a learning quest. Google can be your friend to help you find them, but it may take some work on your part to generate successful search-strings of text.
  6. Hunter64


    Nov 20, 2018
    A Hall-effect sensor might be more suitable.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Yeah, but that would depend on why sensing the moving magnetized needle is important. So far, no response from @seza after the initial post.
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I 'm fairly confident the question is related to his other post here. The coild and magnet form a sensor pair to detect the position of an indicator (arrow).

    @seza : The coil will sense the magnet passing over/across it. The change in magnetic field will create a change in induced voltage which can be sensed.
    But: Once the needle/arrow is stable, no longer moving, the magnetic field will no longer change and consequently no voltage will be induced. So this setup is suitable to detect a change in position, but not the position.
    Insofar the answer by @PETERDECO in post #2 is helpful (although unrelated to the question). Reed switches could detect the absolute position of the indicator magnet.
  9. seza


    Nov 6, 2015
    Sorry for the late reply - guys and thank you all for your input,
    yes as Mr Kapp pointed out, this thread is related to another one,
    where I asked about playing audio file when an indicator arrow/needle point
    to an object on a board.
    I am happy to learn about devices that will do the job even if they are not related to the question
    so thank you PeterDeco for mentioning REED switches, those look interesting.
  10. Bluejets


    Oct 5, 2014
    As already pointed out, it depends on the strength of the magnetic field.
  11. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Hmmm. Let me venture a guess... you have a board game with tokens or objects arranged, more or less in a circle surrounding a central point, wherein a needle arrow, presumably mounted on a pivot to allow the needle to spin about the pivot, can be spun by a player's hand. Eventually the needle will stop and point at one of the objects or tokens, whose symbolism dictates the next move of that player. Sort of like the "Wheel of Fortune" or "Price is Right" television program where players take turns spinning a giant wheel to collect money prizes, or lose turns, or whatever "surprises" come up. You want a "Wheel of Fortune" that plays on a horizontal board, and perhaps has segments that "light up" as the arrow rotates to point at them. The rub comes if the arrow stops between two adjacent segments, with no one segment indicating the "winner" of that spin of the arrow. So, you need an electronic means to eliminate the ambiguity.

    The television Wheel of Fortune uses a ratchet mechanism to ensure the wheel stops at discrete locations pointed to by a fixed pointer. One scheme uses several pegs in each segment that the fixed pointer is forced by the mechanics of the situation to always land between, Thus there can be no doubt as to which segment the pointer has selected, although there is considerable suspense as the wheel slows down and the pointer flaps between the rotating pegs.

    A similarly functional arrangement could be designed into a spinning pointer on a horizontal board game by placing small rare-earth magnets on the board to replace the pegs of the commercial version. Then, after the needle has been spun by hand, it will pass over each of the magnets in turn until it slows down enough to be attracted to and remain over just one of the magnets. Then all you need to do is detect the position of the arrow pointer, i.e., determine which segment it stops on. A rotary shaft encoder could easily do this, or perhaps you could embed sensors under the path of the arrow needle to determine which segment it landed in.

    There are really two separate problems that must be solved to make this type of game both fair and possible, First problem is to guarantee that the pointer will always unambiguously point to one, and only one, segment. The second problem is to identify (electronically, so you can select an audio file to be played) the segment the needle pointer has selected.

    Of course, if I have incorrectly guessed the intent of what you want your magnetized needle to DO... well, never mind.

    But, if I am on the right track, magnetic reed switches can serve both the purpose of ensuring the arrow pointer lands only within discrete areas of each segment, as well as providing electric position indications of the selected segment. The spacing between each reed switch and the magnetized needle will be critical to seeing that the pointer is attracted and eventually held stopped relative to one particular reed switch. This can be accomplished with careful mechanical design and some testing to determine the optimum spacing between reed switch and magnetized needle before constructing the final board. Perhaps you could use a strong plastic, such as Lexan (polycarbonate), to mount and glue all the reed switches into shallow recesses milled into the panel, and to accurately mount a low-friction bearing support for the magnetized needle that allows it to pass over, and be attracted to, the reed switches when it's spinning slows down enough to allow the needle to be "captured" by one of the reed switches. This construction technique may eliminate any requirement to embed strong magnets in each segment to fix the "landing" positions of the needle.

    Building such a device could make for an interesting home board game. Commercial prospects are nil. As soon as anyone who begins manufacturing and selling such a board, they will soon find that there will be bells and whistles added by Asian manufacturers and their product will undersell your product to the point where yours will become unprofitable to manufacture and distribute. Capitalism and cut-throat business practices have now been exported free to the world!

    However, one of the nice things about this forum is it encourages folks to learn how to make "one off" projects and this sounds like it would be a good one. Playing sound bites associated with each segment is innovative. Be aware that by my mentioning this here, the idea will be stolen and implemented elsewhere, but that can't stop you from enjoying your own DIY board game.
    Last edited: Jun 11, 2021 at 2:38 PM
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