laser transmission - 6.5 miles

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moxhamj

New Member
Very nice. I did some laser experiments a couple of years ago. Take a cheap laser pointer and it spreads out to 30cm over about 60 metres. But if you beam it in reverse through a small telescope (I used a monocular - half a binocular) you lose about half the brightness but the beam comes out very parallel. Then some sort of concentrator at the other end - I used an A4 sized fresnel lens - only a few dollars at the newsagency.

With some simple op amp circuitry to give a square wave, and/or modulation at a higher frequency, it ought to be possible to get a good enough waveform for a picaxe serial link.
 
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John West

Senior Member
fred_b said:
Very interesting experiments using a standard laser to transmit audio. The sample music that was sent 6.5 miles had pretty good quality. Very detailed descriptions and schematics...

http://www.maxmcarter.com/lasrstuf/laserexperiments.html
Click to expand...
Yep - I did the bd layouts for it. I'm the old fart in a couple of the range test pics.

6.5 miles was right on the edge of functional range for the system as configured - so there's a bit of sputter in the audio. But at 5 miles it's dead clean and quiet.

I've been considering using a PICAXE to control small steppers to point the thing - but that may be more effort and gear than is reasonable. The whole idea was to see how cheap and easy we could make it and still squeeze high performance out of it. A fun project.

I hadn't considered the PICAXE serial data link aspect of it. I think I'll look into that.
 
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moxhamj

New Member
Re lining it up, I found if you did it at dusk you could see the laser dot in the distance, especially with a telescope. Then just move it around till it hits the target. But I'd be interested to hear how it was done at 6.5 miles.

Re serial data, this ought to be no different to radio links, where you have white noise then you send some 01010101 bits (ascii U) to get it ready then a wakeup preamble and then some bytes. If it is an AC coupled waveform, the UUUU will bias it to the centre point. Maybe use a 1uF bipolar or greencap, and bias the picaxe side to 2.5V weakly with a couple of 10k resistors in series. If the amplitude is right eg a 4V to 5V peak to peak AC signal, and it is biased as above, it ought to go straight into a picaxe with no further processing needed.
 
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John West

Senior Member
The laser is carefully aligned visually in conjunction with a small rifle scope - in this case a $6 air rifle scope.

Once the spot is in the middle of the cross-hairs at a distance all you then have to do is point the scope to where you want to set up the receiver.

As the distance grows the laser spot fully blankets the cross-hairs since the beam diverges with distance. At 6 miles it's about 40 meters in diameter.

We found that with careful pointing of the beam we could go right to the receiving spot with only a bit of "looking around." The beam from a 1 to 5 mW red laser is visible at that distance even in daylight - if the air is clear. Wyoming has very clear - dry air and very few trees and shrubbery to be a bother during testing.

Once there and generally pointing the receiver at the beam we "fine tune" it by carefully observing (from behind the receiver) a wax paper disk that surrounds the receive photodiode - looking for the telltale red dot. Once it's on the wax paper we simply adjust the direction of the receiver tube to center the dot on the detector. (Max comes up with some pretty good ideas.) And finally - we have a small portable audio amplifier hooked up and we do the most precise "tuning" by ear.

One significant factor in gaining distance is the diameter of the "light bucket" at the receiver. For my system (which is similar to Max's) I used a one dollar 8"X10" flexible Fresnel lens. The optical quality was obviously low but it didn't make any real difference in this application. My setup appeared to work just as well as Max's at the 6.5 mile receiving site.

If you have a clear line-of-sight between a remote data acquisition site and your base station a PICAXE and a setup such as this could link data at significant distances - without a license required.

I've been toying with the idea of running a higher powered laser and aligning it using a larger focusing lens so the energy is spread out enough to protect the eye.

There's lots of info on the apparatus (as well as the testing we did) on Max's website: maxmcarter.com

He's a careful and thorough designer and it's a pleasure working with him.

I'd be very interested in seeing the system modified for a PICAXE data link for control and monitoring purposes. Living at the foot of the Rocky Mountains I'm always working on coming up with oddball ideas for long range comms links up into the hills. This sounds like a good one.
 
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