[Laser] Some sort of light-based thingie... (was: 5mw laser "tranceiver" kit)

[C. Turner]

In following this thread, I've become puzzled as to what it's about.  
Interesting ideas, all, but they seem a departure from the intent of the 
O.P.

Were I to design/offer a kit, it would be to provide a reasonably 
high-performance system for a reasonably low cost:  I think that we 
would all agree that there are a number of kits, designs and plans out 
there that do little other than to demonstrate the physical principles 
involved - but fall far short of being a practical device or 
demonstrating the practical possibilities of the medium.  Fewer still 
seem to do even that very well and adding yet another mediocre box of 
parts to the gallery would be of questionable value.


A few random thoughts on what I would hope to see on something that 
would be peddled to the masses:

- Visible light only.  Forget that IR may travel better through 
haze/dust/etc.  In reality, the difference isn't remarkable unless you 
get way down in the spectrum - which rules out things cheap and simple.  
Because IR is invisible, adjustment/alignment is somewhat complicated as 
everything must be done with cameras and/or IR-sensitive phosphors..  If 
it isn't easy to use, people won't!

- Forget daylight use.  While interesting, it would first require that 
one has a system usable in the dark as well as experience in making it 
work.  Truly "daylight blind" systems are possible, but to work well 
they require some esoteric devices if they are to be used over more than 
very short distances - unless you revert to Alexander Bell's methods 
(e.g. modulated, reflected sunlight.)

- PWM versus current modulation.  PWM is dirt simple to implement - just 
a couple of transistors (or one op amp section) and a 555 will do it and 
drive a pretty hefty LED by itself!  PWM will also work with both LEDs 
and lasers, as opposed to attempts to voltage/current modulate them:  
The former doesn't work well with any solid state emitter while the 
latter is most appropriate for LEDs.  (As Jim pointed out, attempting to 
current-modulate semiconductor lasers is pretty "iffy".)

- Lasers are Cool!  They are also a pain in the @$$ to aim and anyone 
who has attempted to do so over any sort of distance will tell you so!  
There are ways to do this as I've detailed on this web page:
http://modulatedlight.org/optical_comms/using_laser_pointers.html
Doing this requires a bit of planning and forethought in the equipment 
design and no practical system should be contemplated without 
considering these or similar techniques.  The intrinsic, wider beamwidth 
of LED-based transmitters makes them far easier to aim and at longer 
distances, the performance of a Luxeon I/III/Rebel will easily exceed 
that of an inexpensive Class IIIa laser pointer in a properly-designed 
system.  (I've done it, compared the two, and posted results on the web.)

- A reasonably-sensitive detector.  Again, unless your aim is to simply 
shoot light across the room, you'll need something far better than a 
solar cell/phototransistor.  There are already a number of simple, 
proven designs that, when coupled with even a modest-size lens, can 
offer decent performance and it would be nice to see just one kit that 
made a reasonable attempt at doing so.

Now, I'm all for experimentation:  I think that coming up with some 
interesting schemes to achieve descent daylight performance would be 
fun, as would be a means of coming up with the simplest-possible 
short-range (e.g. low performance) optical communications system.  It's 
just that it strikes me that one would want to design into it a few of 
the basic principles that we know would contribute to its success and 
performance rather than haphazardly assemble parts and hope for the best.

73,

Clint
KA7OEI