Wireless Data Connections Through Light

When wired networking or data connections can’t be made, for reasons of distance or practicality, various wireless protocols are available to us. Wi-Fi is among the most common, at least as far as networking personal computers is concerned, but other methods such as LoRa or Zigbee are available when data rates are low and distances great. All of these methods share one thing in common, though: their use of radio waves to send data. Using other parts of the electromagnetic spectrum is not out of the question, though, and [mircemk] demonstrates using light as the medium instead of radio.

Although this isn’t a new technology (“Li-Fi” was first introduced in 2011) it’s not one that we see often. It does have a few benefits though, including high rates of data transmission. In this system, [mircemk] is using an LED to send the information and a solar cell as the receiver. The LED is connected to a simple analog modulator circuit, which takes an audio signal as its input and sends the data to the light. The solar cell sends its data, with the help of a capacitor, straight to the aux input on a radio which is used to convert the signal back to audio.

Some of the other perks of a system like this are seen here as well. The audio is clear even as the light source and solar cell are separated at a fairly significant distance, perhaps ten meters or so. This might not seem like a lot compared to Wi-Fi, but another perk shown is that this method can be used within existing lighting systems since the modulation is not detectable by the human eye. Outside of a home or office setting, systems like these can also be used to send data much greater distances as well, as long as the LED is replaced with a laser.

38 thoughts on “Wireless Data Connections Through Light

    1. Unless doing it just because, I hope the ronja parts list is much cheaper nowadays; given that commercial, directional WiFi from companies such as Ubiquiti don’t cost a lot more and cheap chinese ones that don’t care about the FCC are roughly same price.

      And don’t forget Forrest Mims.


      Pretty sure he had a couple of simple light based projects for voice/audio similar to the one described.

          1. Line of sight for microwave is dependent on the elevation and terrain. Terrestrial microwave routinely works out to 100+ miles (offshore links) off of Norwegian cliffs and mountains near Rio de Janeiro.
            Microwave works reliably with 2 watts over a distance of 23,500 miles. Called VSAT.
            Limitation on light comms is rain, fog, dust. Typical reliable range a few hundred yards. If you don’t mind unreliable weather outages you could probably get 70 miles out of light, maybe more depending on terrain clearance, path refractivity and Fresnel zone clearances. (with laser).
            For a blazingly fast link indoors across one end of a stadium to the other end? Yeah. That.
            One of the issues of microwaves is solar outages where the sun creates microwave noise and lines up with the radio path.
            The sun makes light, too. Could be a problem on east-west terrestrial paths.

      1. He did, his little books inspired many to get the iron out and try stuff, still have some circuits on veroboard, his circuits were well tested so if it did not work it was your fault. surface mount killed the magic for me.

      2. Yes, Mr. Mims did have several projects for communicating over light. They were the basis of several projects I helped my kids with when they were younger — I think the most complicated one (if you could call it that) was a pair of LEDs hooked to serial ports, which were configured with SLIP. Dirt simple basic network connection.

    2. I’ve been wanting to build a Ronja system since reading about it (probably /on Slashdot) back in 2005. Has anyone actually built one from the their design? I figure Hackaday is the best place to ask :-)

    1. Indeed, and there are a variety of commercial products for this very purpose. In general, up to 100-300m depending on optical conditions, you can get a reliable link at Ethernet-like speeds.

  1. Also, the light can be FM modulated. This improved sound quality and suppressed hearable fluctuations in brightness compared to a pure AM when I sent the light through a linear flow water jet: https://www.youtube.com/watch?v=SmNLuugyjc0 (not my Youtube channel)

    This is a demonstrator I built for the open house event used to attract young people to the topic of fiber optic communications at the TU Dortmund university at the Chair of High Frequency Technology.

  2. Hams have been doing this for the better part of a century. Before WW2 in the Dec 1943 issue of Radio Craft there was an article on the German military adaptation of the technology with the Lichtsprecher (light talker)
    The all-time distance record for terrestrial optical communication with speech modulation was set on 3-4 May 1963, when a 632.8 nanometer helium-neon laser beam was transmitted 118 miles by W6POP and W6QYY, from a point in the San Gabriel Mountains near Pasadena to Panamint ridge near Death Valley, California
    Hams have even do full bandwidth video now. On July 23 2013 the Laser Amateur Television distance record was set at 118.4km from DL9OBD,DJ1WF ,DD0DR, DK3BM, DG1YHC and DB4QM. This was a full duplex video link!

    There has even been some amateur research on high altitude sodium layer reflection and non los Rayleigh scattering optical communication systems.

        1. Read the sentence again. “Before WW2 in the Dec 1943 issue of Radio Craft ”
          When the article was published in December 1943, WW2 was almost four years old.
          It is not talking about dating the invention, it is about the issue of the magazine article.

      1. That is a very Eurocentric view you have there. You are ignoring the Japanese invasion of China. As well as 1935 attack on Ethiopia by Italy. You could even clam that Dec 7 1941 could be seen as the official start of WWII since that is when the last of the major powers entered the war. My honest opinion is that WWII started the day WWI ended.

        1. I nearly forgot how some hackaday readers are..
          I should have been clearer that it was 1935 when Carl Zeiss began production of the Lichtsprecher and the first english language mention of the unit was in 1943.
          I was thinking the post was already getting too long and people might have more insight to understand we were talking about free space optical communication not geo political events.
          Here is a radical thought, try to add to a discussion with subject matter related to the discussion at hand. For example you could add to discussion if you mentioned that the amateur radio satellite AO-40 had a laser telemetry system down link in 2000.

          1. Here is a radical thought, next time get your chronology in order.
            Something like “The German military was already doing this before WW2, as described in a December 1943 issue of..”
            Wouldn’t have made the post that much longer, would it?
            The way you phrased it is just plain wrong.

  3. i was kind of disappointed to see 10 meters.

    i am only recently coming to grips with the implications of having more than one bit on a line at the same time…you know, when the bit period is shorter than the distance divided by the speed of light. you put a 1GHz signal on a 10ft line and suddenly you’ve got 10 bits marching down that line together at 1ft/ns, and any reflections from impedance mismatch are going to kill it. kind of stretches my mind, maybe it can’t quite fit in there yet.

    but i imagine lasers don’t have that problem, at least not in the same way…right?? anyways i wouldn’t’ve minded reading a little article about that :)

    1. Reflections can cause problems in low-loss lines because they do not get attenuated, and so can interfere with the desired signal. In an optical link, reflections might occur, but in free space that inverse-square law really works to limit how much a reflection can interfere.

      That inverse-square law also is a pretty hard obstacle to high bit rate free space communications, because you need to receive and detect enough photons above ambient background to unambiguously recognize a bit. So you need low ambient background light and/or excellent narrowband optical filters, and high-gain ‘antennas’ (telescopes).

      In an unconstrained path, where the emitter is transmitting over 2pi steradians, and the receiver must accept input over a wide range of angles, you don’t have the option of high gain optics.

      So, work it out: how bright must a light source be, for a receiver to unambigously detect a bit per nanosecond, ten meters away?

      Say you need 50 photons to statistically detect a bit against a dark background (implies a windowless room, no other light sources like power indicator LEDs, and a very good low noise detector).
      At 2 eV per photon (average energy of visible light), that’s 100 eV, or 1.6e-17J per bit
      Doesn’t sound like much energy, right?
      At a nanosecond per bit, that’s still only 1.6e-8 watts. 16 nanowatts. A tiny amount of power.
      If the receiver is a square millimeter of silicon (pretty big for a gigabit-capable receiver, but probably still OK), that’s still a measly 16 milliwatts of light per square meter.

      At a ten meter range, that is a 630 square meter hemisphere, so the light source must emit 10 *watts* of light power in the detectable spectrum of the receiver. Roughly speaking that’s a hundred watts of electrical input power for that gigabit speed at 10 meter range.

      Wifi isn’t looking so bad any more.

      Free space optical links are always crappy in bits per second per watt.

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