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Thread: Slow I2C\UART over stereo FM?

  1. #1
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    Slow I2C\UART over stereo FM?

    I have an application where it will be cheap and convenient to use a low power FM receiver to manipulate a remote unit. The remote unit is to be cheap and low power. I'm looking into using a stereo FM channel to supply I2C signals (SCL\SDA) to a set of peripherals so I won't need an MCU. Specifically, I want to set digital potentiometers such that LEDs turn on and off. It seems like somebody may have proven this out already, but all the searches for FM\I2C get hijacked. Any hints?

    I'm also looking into receiving data via FM as well. I can see in the below thread that people have made I2C slave mode work. Theoretically, a stereo channel could do unidirectional downlink. Similarly, a stereo channel could also provide two UART Rx channels. I'm looking for projects where people have done this, but I can't find anything yet. This seems pretty simple, but I'm suspicious because I see no implementations.

    Given a Teensy4.1's abundance of communications channels, I could really get a lot done with a bank of cheap receivers.

    https://forum.pjrc.com/threads/57861...ve-on-Teensy-4

  2. #2
    Senior Member+ Frank B's Avatar
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    Quote Originally Posted by kdharbert View Post
    but I'm suspicious because I see no implementations.
    Because it is not allowed.
    You can use a cable, light or ultrasonic.
    Or just use some of these commercial units you can buy everywhere. You can even use a ESP8266 .. you don't need a teensy in the receiver then. Very small.

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    I don't follow. You can get low power FM transmitters that are legal. They go about a block on an empty station and to the next room on a populated station. Am I missing something?

  4. #4
    Senior Member+ Frank B's Avatar
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    Yes i was talking about these transmitters. But don't build your own. You asked for that in the other thread.
    I2C is bidirectional.

    But as said - these ESP8266 are ultra-cheap, work perfect, and have a cpu on board. I'd just use these as receivers, and connect one to the teensy "master".
    Then, you have a very fast, reliable, bidirectional connection.

  5. #5
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    I'm still confused on the legality you're referring to. All that is required is sending stereo audio.
    Although I2C is bi-directional, its not required in this case, so stereo FM works

  6. #6
    Senior Member+ Frank B's Avatar
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    You can buy stereo audio transmitters only for the common broadcast frequencies. Around 90MHz. That you can buy them does not mean that they are allowed everywhere. And if, for AUDIO only. No Data. (RDS perhaps, embedded..)
    The allowed transmitters use a very very low tx power. Enough for your car. Cigarette-lighter->Radio. Not more.
    Everything else is illegal. Yes you can buy illegal transmitters in china. They don't care. YOU will get the trouble, not chinese shops.


    There are other boards that are allowed. They are built for data.

    But not FM Stereo.
    Last edited by Frank B; 07-28-2021 at 10:01 PM.

  7. #7
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    Interesting. I was aware of no rules forbidding data transmission.
    Where can I read?

  8. #8
    Senior Member+ Frank B's Avatar
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    Use Google. This is my last post here.

  9. #9
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    Second lap through google was also unsuccessful.
    I did turn up references to what you mentioned about vendors simply lying about FCC certification.
    Good info.

  10. #10
    Senior Member DD4WH's Avatar
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    Quote Originally Posted by kdharbert View Post
    Interesting. I was aware of no rules forbidding data transmission.
    Where can I read?
    Come on, its not so difficult to find . . .
    Try Title 47 of the Code of Federal Regulations . . .

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