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What is the best method for having recorded audio play when in the vicinity of ferrous metals?

I am working on an engineering project and I am looking for ways to have recorded audio play when in close vicinity to ferrous metal. Specifically, I’d want the method to be as compact as possible. I’d assume companies sell sensors that can detect these metals through induction. Also companies probably sell devices that play back audio on cue. What would be the best way to tie these things together? Specifically, what types of sensors are small(MEMS level) and cheap that could detect the metal and transmit a voltage to an audio device(as small as possible, but capable of producing audio of recorded voice clearly and at an audible volume) and use only a minimal amount of power(say the amount a AAA or smaller battery could produce)?
After seeing the first response I’d like to add that the metal will range between 1-5cm from the sensor. I’m not looking for some impossible device.
Let’s also assume that the metal are disc shaped pieces ~.5-1cm thick with a diameter of ~2.5 cm. And you have at least a .5-1 seconds for detection.

You are "researching" a metal detector with built in mp3 player??????????

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Wow. And I thought I had seen it all.

OK… how close can you get to the ferrous metal parts? And how big are they? And how much time do you have for the detection?

In general it is hard to detect small metal parts from much further than a few times the size of the antenna of an RF or pulse metal detector. That is physics, so MEMS technology will not help, no matter how cool it sounds. Now, you can detect large metal parts with a small antenna just as well as you can detect small parts with a large antenna- it’s just a matter or reciprocity for electromagnetic waves… one thing has to be large so the other can be small.

You can, of course also detect small parts with a mm radar… because it has a narrow beam… but those are neither small nor are they a low power solution.

Good news for you is that the mp3 player part is trivial. So you can factor the problem into two parts: a hard one that you defer to another student and a trivial one… how to turn on an iPod.

:-)

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  1. amansscientiae says:

    You are "researching" a metal detector with built in mp3 player??????????

    ??????????
    ??????????
    ??????????

    Wow. And I thought I had seen it all.

    OK… how close can you get to the ferrous metal parts? And how big are they? And how much time do you have for the detection?

    In general it is hard to detect small metal parts from much further than a few times the size of the antenna of an RF or pulse metal detector. That is physics, so MEMS technology will not help, no matter how cool it sounds. Now, you can detect large metal parts with a small antenna just as well as you can detect small parts with a large antenna- it’s just a matter or reciprocity for electromagnetic waves… one thing has to be large so the other can be small.

    You can, of course also detect small parts with a mm radar… because it has a narrow beam… but those are neither small nor are they a low power solution.

    Good news for you is that the mp3 player part is trivial. So you can factor the problem into two parts: a hard one that you defer to another student and a trivial one… how to turn on an iPod.

    :-)
    References :

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