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Laser Ethernet Data Link

Sigurthr

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Is there any reason you have to use a laser? Optical data transmission can be done with high powered LEDs as well where the dangers and most prominently the focusing and alignment issues are greatly relaxed. I've done free space optical communications (laser data links) before and found that at any reasonable distance (even 75ft) the alignment becomes a nightmare even with "stable" equipment setups (professional tripods, etc). Unless you've got a precision gimbal or other XY adjustment methods you're going to have a very hard time with using a laser link.
 



perfo

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Well no they haven't got to be lasers but the LEDs would have to be fast enough for a 10 mbps lan and be able to penetrate 30m of water. I did put a couple of high power LEDs at the bottom of the pit just to get a comparison between them and the red laser and I think they worked as well as the laser but with my limited knowledge I didn't think these standard type LED a would modulate quick enough. But yes I'd be very interested in an LED solution. Maybe a couple of high power LEDs ? I guess I would need to ask for assistance in providing a high speed driver circuit for them as the media converter is bound not to be able to supply them with enough oomps. Just out of pure luck you didn't mean 75 ft in water did you ? The commercial laser links I've seen are way too expensive for me to be mucking about with , especially drowning them :)
Aligning funnily enough isn't the major problem for me as it is relatively short distance and the spot size is pretty big by the time it gets near the surface.
couldn't find any circuits or other on the interweb concerning video transmission with LED's or even doing a network bridge (essentially what I'm trying to achieve) but I guess it's out there somewhere.
 

BShanahan14rulz

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Usually the thing that limits your bandwidth will be the driver you use. The LED or laser diode can switch on and off super fast, no problem.

RONJA - Wikipedia, the free encyclopedia
Home
^ this may interest you. An old, point to point open source optical transmission project that started in the czech republik.
 

perfo

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Interesting post on the RONJA thing. I'll have a good read of that this weekend. I had seen it in Wikipedia but missed the bit where is said all the designs and plans were on the site. Thanks.

I tried to do a bit of research and I thoought soem LED's had doping ont hem that slowed down the maximum speed. But If you are saying any LEd will be capable of the speeds needed for Ethernet transmission (which I think is about 2.5Ghz) then I won't disagree with you as I really know very little about these things at that level. I've only used LEDs for indicators and that's about it.

My Media converters have turned up today and I've had one apart. It isn't fantastically obvious which terminal goes to which LED as there is about 8 connection to a little metallic box with the TX and RX inside. I will de-solder it all and risk damaging it if need be but I'm hoping someone on here may be able to point me in the right direction for a circuit diagram or something ..
They are Allied Telesyn international model AT- MC101XL ..
Thanks.
Another question if I may is is it possible to make a light guide for a laser or LED that is rather than my laser pointing to a floating surface receiver maybe it could point to the wall of the pit and have a light guide going top to bottom and the optical receiver on the top of the guide ? I know this if it works would only provide one half of the Ethernet link but maybe it would help.
 

BShanahan14rulz

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Well, now that you mention it (and to be fair, you mentioned this in the OP), the GHz range might be a bit high for a generic or typical LED. I would guess that the higher wavelengths might be required, and my train of thought is that since each phonon formed from a smaller transition, perhaps these transitions take shorter time. Also, must be some reason why they use red and IR light for data transmission.

If your transmitter will be switching at several jiggahurts, you might need to slow your expectations or verify that the LED or laser you choose specifies what its time-on and time-off is.
 

The Lightning Stalker

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There are a few reasons why IR is used over visible. The low voltage drop of an IR LED/diode is much
easier to drive with the high speed <5V chips out there. It also cuts down on losses in the voltage
regulators since the input can be lower. Another reason is that IR LEDs are a more mature technology.
The first LEDs were IR. High power IR LEDs can be had very cheaply. Also, since no one is going to see
the light anyway, it does not need to be a wavelength that is visible to the eye.
 

perfo

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My reasoning for not starting with the IR route was due to IR being the worst frequency for transmitting through water but that’s not to say if my LED torch with white LEDs showed a faint glow over the distance then high power IR may do it. If they are cheap enough then maybe a few IR LEDS and a biggish lens of some sort may do the trick. Again I wouldn’t want to get in to enough power on the IR to damage anyone.
Could a high power IR LED cause retinal damage?

This leads me to another thought. I’ve had a good look at the RONJA link.
I found it a bit difficult to navigate around the site I read things like Ethernet cards for the PC and RJ45 etc . So to me this means the card is converting the Ethernet and the RJ45 is the connection to the transceiver modules, but there are three boxes as part of the transceiver with a lot going on and I’m not sure what it’s doing.

Please correct me if I’m wrong (I expect corrections :) ) The signal going down an Ethernet cable is simply a fast digital stream of 1’s and 0’s? So all I need is a driver to turn the LED on with the 1’s and off with the 0’s ? Am I missing something here? This gives a huge advantage over trying to package it up with a WIFI signal as there’s no high speed carrier, is this not the case? The RONJA seems a lot of electronic for a driver circuit, I appreciate that maybe the media converter is doing a lot more than simply converting I guess it is doing its own error detection and the like. But for my use if it was a simple driver and LED transmitter with photo diode receiver the components either end should do the error correction and the link is undetectable.
 

Multimode

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Maybe be take a step back -
Perhaps it is less of a technical challenge to design a system that prevents a fixed ethernet cable / fibre optic from being snagged than designing a reliable laser link ???

ATB
MM
 

perfo

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The best one I'd come up with was a floating platform with a cable drum and WiFi access point on it with a motor doing the rewind but of course the motor has to current sense to keep the cable taught but not break something when the crane stops. I thought of steppers with either a tension switch stopping the steeper or current sensing and the stepper going in to lock mode when not moving. The simplest one I've come up with so far is a geared DC motor and a few magnets on the shaft and on the cable drum so it slips when the crane stops but always keeps a tension on. I haven't found an easier way to do it but with winches and drums and motors etc there are probably more thing's to go wrong than with a light link. At least the light link either works or not. I did spend the first part of my project trying to think up a way of doing a direct link but haven't found anything that I'm happy with. The crane gets used for all sorts and the pit has different things put in and taken out so any drums etc would have to be very easy and quick to put on and off and fool proof. I also hoped to leave the cameras on the beam once set up for the right position and thus maybe the drum should not go on the float on the surface but on the beam and the float simply has a WIFI AP to get the signal back in air. This idea works nicely except for the cable retraction.
 

perfo

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Reading this document

http://cp.literature.agilent.com/litweb/pdf/5989-7528EN.pdf

It appear that 10base t is literally 10mhz I know that may seem obvious but I had always thought more was going on than a simple one pair of cables transmitting and one receiving thus the four cores of cat5e

100base t uses is 100mhz but uses more cores at the same time.
So maybe I could get away with the simpler 10Mb/s of the 10base t system.
Any suggestions? Could I put a few transistors on the TX pair of cores and aim it at a simple receiver circuit (same in the other direction) ?
 

Sigurthr

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Most simple cheap red laser diodes can be modulated above 400MHz rather easily. I've seen a low power blue laser diode get modulated at 15MHz as well with nothing special required. As for LEDs the trick is having a driver that can supply such short pulses of high current. It can certainly be done but you either need power bipolar transistors or VERY fast mosfets. In either case stray inductance and parasitic capacitance will be the biggest challenge. The inductance will cause spikes and the capacitance will round off the nice square edges. Together they'll induce ringing.
 

The Lightning Stalker

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Could a high power IR LED cause retinal damage?

They most certainly can! Since the light is not highly collimated, though, the power levels
fall off quickly with distance. As the light spreads out, the power density falls and
becomes less dangerous. It depends on the emission angle. I have been using some
SFH 4545 which have a 10° angle and the datasheet actually carries an eye damage
warning. I think you have to actually have to put them right up to your eye to really cause
damage, though. I'm still not taking any chances. I don't ever point them at my (or anyone
else's) face.
 

perfo

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I guessed the IR LEDs might have the potential for damage, especially as I'll be using some sort of lens to get a bit more distance.

Would anyone have any suggested circuits for driving with laser or LED from an Ethernet signal ? The simpler the better of course. Unless someone can put me right I'm thinking 10Mhz should do it and I'll try it on 10 base t.

Thanks...
 

The Lightning Stalker

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A good place to start would be to feed the signal through a resistor into the base of a PN2222A or similar.
Then on the output side tie the emitter to ground and the LED on the collector in series with a resistor.
Keeping it simple will ensure operation at higher speeds. Put a good size cap across everything. At
10MHz there shouldn't be much of a problem.
 

perfo

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Cap across the supply to the little circuit ? Do I need to worry about constant current or any of that malarkey ? I suppose it would be ok to parallel LEDs providing I adjust the resistor value. That seems a nice simple (within my range) type of circuit . Thanks matey. What is the RONJA circuit doing more than this in it's three boxes ?

Sorry to probe your mind further but would you have a similar suggestion for a receiver circuit? Whilst the fundamental's of these things I can handle I have no practical experience of what components behave properly at these speeds. Thanks again
 

Sigurthr

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The receiver won't be so simple as the sensor capacitance will be an issue, so you'll have to look for a photodiode with rise and fall times (added together) less than 1/10th of a period of your maximum frequency.
 




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