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Comment by codezero | Hacker News Reader

In college for a project I wanted to encode messages over small brightness variations in light bulbs. It's a pain to deal with AC, so I ended up doing it with a laser pointer. It turned out to be stupidly easy with a laser pointer and photodiode attached almost directly to a pair of PC speakers. All these devices have a +5v DC signal. I could attach my iPod headphones to the laser pointer directly, then the photodiode to the speakers, and boom, music at about 100 yards, with almost no quality decrease, though I didn't do anything quantitative :)

I've always wondered what places communication started getting hidden since we went online and no longer rely on things like numbers stations or drop boxes (though I assume dead drops still exist)

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A long time ago back in college our team designed a free space optical gigabit digital link via laser diode and photodiode. We were transmitting I think 1 Gbps easily over maybe 50 meters. We didn't have enough time to polish the system or test faster speed because the sponsoring corporation took our design back to their RD group.

Twenty years ago I worked at a place that had offices in buildings across the street from each other and had a laser network link between them in case the link through our ISP stopped working. I don't recall if we ever had to rely on it while I was working there, but I remember the guy in charge of monitoring it used to complain that it didn't work on foggy mornings. One end of the link was on the 21st floor, and we got fog at that level a lot more often at that height than at ground level.

I bet this was a microwave dish, super common to connect buildings with line of site., and prone to fog and weather :)

The Wikipedia entry for FSO networking is pretty misleading. There have been commercial products on the market for quite a while.

It's more likely that it's a microwave link, though, right? Hence the betting.

I did no research, so hold no strong opinion. I’m in favor of lasers whenever possible. :)

Li-Fi projects are the best if you want people to get into information transmission. It's simple an cheap and easier to grasp than any radio based transmission. Two diodes (you could even use a LED as sender AND receiver!) and you are ready to go.

K what? That's amazing! Apparently I need to learn more about lasers.... haha :) I guess it's pretty "fragile" since the slightest misalignment of the source laser & destination sensor would mean a total loss of signal, right?

Given that the laser forms a spot, minor misalignments are very easy to debug (just line up the red spot with the sensor again). Also given a laser with sufficiently wide spread, minor misalignments would manifest themselves as volume loss.

Not great if you don't have easy access, but reliable enough for e.g. a dorm room or a living room.

This was a lab we did in an intro EE class in college and it worked wonders as a "whoa cool" demonstration with very basic EE knowledge required.

You’ll probably also need to debug it often on warm summer evenings.

You should find out how the starlink network will communicate if you're worried about accuracy.

Seeing as how Starlink doesn't have this capability, it would probably be better to study any other instance of free-space optics, many of which exist.

The other commenter is right, store bought lasers have a descent spread over a few dozen or hundreds of feet. And the signal is still pretty readable to the receiving photodiode.

Free space optical system *intentionally" spread the laser beam using a lens. The light beam still consists of parallel rays, and when they hit the receiving lens they focussed again on the receiving diode (or whatever sensor). The idea is to accept that the beam will be disperse in the air, or due to variation at the transmitter, but still to catch enough of the transmission. https://en.wikipedia.org/wiki/Free-space_optical_communicati...

Awesome. Thanks for the wiki link, will check that out.

You can also send the sound via the laser directly to a MEMS microphone, if it's exposed: https://www.wired.com/story/lasers-hack-amazon-echo-google-h...

Fun fact: This is how TV and telephone once worked. Except with microwaves.

https://www.wired.com/2015/03/spencer-harding-the-long-lines...

Could you point out more info about this? I'd like to build something similar, but I'm not sure where to start. It sounds like such a cool project.

http://ronja.twibright.com/

Is an old project, that I have memorized the bookmark ('Optar', is another project somewhere on that page that puts a multi-kb 2d barcode data on a printed page to be reliably scanned in, even if damaged)

I wish I could, I'll see if I can dig up my old report from class, but that was from 2007.

Very simply, I literally just wired the laser in series with the headphones, like, attached directly to the battery of the laser pointer so the audio +/- 5v would jiggle the laser's brightness around that value (the laser was also +5v so this "just works" if I recall. I might have had a resistor in there, and at some point, current leaked back into my iPod and fried it :)

On the receiving end, same deal, PC speaker audio jack attached to a photodiode and power.

It's remarkably analog, but it's also the same reason that things like the now anachronistic CD-player cassette adapter work, throwing 5V AM signals around :)

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