Visible Light Communication

Discussion in 'General Electronics Chat' started by j3r, Oct 17, 2011.

  1. j3r

    Thread Starter New Member

    Jan 9, 2011
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    Hi all,
    I'm experimenting Visible Light Communication. A basic setup would be having a LED (white) with a driver and a photo-diode circuit.

    For example i would like to send this string"HELP". In contrast a binary steam would be send by switching on-off the LED (too fast for naked eye to see).
    The photo-diode would be like a receiver.

    I'm pretty weak with circuit design.

    From my idea, it would be connecting a ADC to the photo-diode.
    Anyone has better idea of making this circuit???

    Many thanks!!

    My final implementation would be leveraging the TCP stack for data transmission.

    How can i covert the output from the ADC to be compatible with a TCP stack.

    It's just another medium used, besides Wi-Fi and Ethernet (RJ45)
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    First of all, I don't have the answer to your question.
    High-speed visible light / or infrared light communication is something I'm looking in to as well.

    There are a few things to consider:

    What will be the distance between transmitter and receiver?

    How are you going to address the problem with ambient light shining on your photo receiver?

    You wrote something about WIFI/Ethernet replacement. Does that mean you are looking for data transfer rates up to 1MBit/s to 100MBit/s ?

    First of all, IMO to decrease ambient light sensitivity you would need a carrier frequency that you modulate with your data stream on the transmitter side. Then on the receiver side you need a bandpass filter to extract the modulated carrier frequency. Much like the IR-transmitters/receivers used for TV sets etc, only at a much higher frequency.
    The transmitter part should be the easier part, but I did not yet find out what optical component for the receiver to use, this part needs some experience and/or experimenting. :rolleyes:
     
    Last edited: Oct 17, 2011
  3. colinb

    Active Member

    Jun 15, 2011
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    The open hardware Ronja Metropolis does 10 Mbit/s full duplex at a range of up to 1.4 km using red LED light.
     
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  4. colinb

    Active Member

    Jun 15, 2011
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    I think it will be simpler if you can avoid an ADC and detect binary transitions in hardware. Then your digital circuitry or MCU can decode the bit stream. Look at Manchester encoding for a good way to encode bits onto a light stream. Look at infrared remote control protocols for an example of how you can encode data into visible light -- infrared remote controls modulate the data bits onto a higher frequency carrier for noise rejection.
     
  5. j3r

    Thread Starter New Member

    Jan 9, 2011
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    0
    High bandwidth is achieved by fast switching of led which where todays technology is evident.

    If u look at the electromagnetic specturm, i think its 30 to 70nm. Infra doesn't fall im this category. Hence any LED eg, the ones used in ur celling, table led lamp, ur tv, laptop screem can be used to tramsmit.

    As for ambient light, there are few modulation techniques used. OFDM is one of them.

    Bandwidth can also be increased with LED array.

    As for performance, some mnc also achieved 500Mb with a distance of 5km.

    Gigabit transfer is achievable.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    That's a nice link. I'm going to have a read through it...

    edit: Well , it isn't exactly what I was looking for, maybe it's okay for the OP. For really long distance like the one they propose you could also use a modulated laser diode. I used them for this purpose to control a TV-set from a few hundred meters distance...

    I was more looking for In-Room diffused light communication... But it was an interesting reading though.
     
    Last edited: Oct 17, 2011
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This seems a very interesting subject. The advent of LED lighting and high efficiency LEDs generally must make this quite a possibility, though I wonder if the phosphors in some "white" LEDs may smear out some of the modulation - anyone know how signicant that may be?

    I would think that hundreds of Mb/s on diffused room light would be very challenging though. A fibre-optic receiver to do that might need less than -40dBm (100μW), but that's with a pure uncontaminated signal, concentrated onto a small photodiode in a receiving amplifier node with minimal stray capacity. A large area detector, plugged into a separate amp in diffuse illumination with lots of cr@p from other sources to contend with? A wideband integrated detector / amp would be interesting, if anyone is making one suitable.

    200Megs though? Bonne chance...
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    To the OP: The output received directly from a photo-diode or even a photo-transistor will be weak unless you can get a very strong light to fall on it. The signal may be too weak for your ADC to respond to. The absolute minimum hardware would most likely require an amplifier before any attempt to digitise the signal.

    Lower bit rates may be practical with an ADC, but higher rates would require very fast ADCs and I would think that a hardware regenerator is going to be cost-effective beyond some rate or other. Of course, this is an old man talking. Perhaps these days the DAC-and-DSP route rules the roost up into the Gb/s?
     
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