[Laser] Laser comm stuff

Fri, 18 Jul 2003 09:05:09 -0400 (GMT)

Interesting web-page.  

I'm a new member of this list, and I'm just now geting in to
playing with laser communications, primarily base band MCW.

I've built a laser transmitter, although I've taken a somewhat
different approach than the one on your web-page (where you use
a 4MHz crystal, a 4060 oscillator/divider chip, and the MOSFET
driving the laser pointer).  For the transmitter I've built,
I used a 555 timer chip driving the laser pointer directly.
While this doesn't have the long term frequency stability of a 
crystal controlled source, it does allow me to vary the frequency
of the transmitted signal.  I've chosen 800 Hz for the 
modulated frequency since that's a little farther up away from
the harmonics of the street lights, and is a little closer to
the frequency normally used to listen to CW.

Oh, one other minor change I've made is that I've tossed a 
silicon diode in series with the laser.  Since I'm driving the
system from a five volt supply, and since the laser pointer 
really expects 4.5 volts or less, and since the 555 doesn't
drop much voltage at all, the silicon diode soaks up a bit of
the voltage to keep from overvoltaging the laser pointer.  Ok,
so it would probably work without it, I just feel a little
safer with it in there.  I think I opted for a 1N400x series
diode.

One of the problems with using a 555 is that it can be difficult
to achieve a 50 percent duty cycle.  Additionally, if the 
frequency is made adjustable, you almost have to vary two 
different potentiometers to adjust the frequency and keep the 
duty cycle at 50 percent.  I opted for the two potentiometers,
but, after thinking about it, I wish I had tossed in a flip-flop
to divide the 555's frequency by two.  That would let me run
the 555 asymetrically (which would simplify the frequency 
adjustment controls), yet still have a 50 percent duty cycle
at the output of the flip-flop (and, since I'm running everything
in the transmitter off of 5 volts, a TTL chip would work well,
and should have the current capacity to drive the laser pointer
directly).

As for the receiver, my design is pretty conventional, except
that I'm including a bandpass filter.  I'm using a PIN photodiode
as the receiver (probably behind a three inch lens, if I can 
get the mechanical mounting worked out), driving a TL082CP 
analog integrated circuit as the amplifier.  Since there are two
op-amps in a TL082CP, I'm using the second one as the active
element in a bandpass filter.  What's good about the design is 
that I can vary the passband of the filter by just varying
one resistance value.  My initial target is for a passband of
100 Hz.  I don't want to make the first design too narrow banded
to minimize problems acquiring the signal.  Plus, I plan to
have the filter switchable so that I can run the circuit without
it.  Thus, I have a gain factor of one for the filter so that 
when it's switched out of the circuit, the signals are roughly
of the same amplitude.  The filter should help me exclude the
street light harmonics, and result in a cleaner CW signal.

For the output, I'm using a conventional LM386 IC amplifier
driving either a small speaker or headphones.

While I've finished the transmitter, I've only prototyped the
receiver on a breadboard (and, it works, mostly).  I'm in the 
final stages of moving the receiver to a printed circuit board.

I'm not planning on setting any record distances with this; I'm
more intending for it to be a show and tell, proof of concept
design that I can exhibit to some of the local clubs to get some
interest going in the highest frequencies.

Dave
WA4QAL

-------Original Message-------
Message: 1
Date: Thu, 17 Jul 2003 21:33:45 -0400
From: "Andrew T. Flowers, K0SM" <[email protected]>
To: [email protected]
Subject: [Laser] Laser comm stuff
Reply-To: [email protected]

I haven't seen much activity on the list as of late, but I figured I 
would let folks know about some stuff I have been working on.  First of 
all, I've completed a couple of baseband receivers and transmitters, 
which I'm hoping to get on the air for our upcoming amateur radio 
contests in August and September.  I've put the schematics on my site, 
except for the LM386 audio amplifier.  You can get that straight for the 
datasheet--just do a google search.  There are pictures of the 
containers and how it mounts in a 4" tube with a 4" lens.  There's also 
a tunafish recipe...you'll understand if you see the site.  I'll be 
updating the site as I get stuff completed and get a chance to take a 
pictures of it.

One thing that I found interesting was the AF frequency spectrum of city 
lights.  I made a few recordings of Rochester's lights scattering off 
of  low clouds and calculated the power spectrum using an FFT.  It shows 
that

1) city lights are roughly square-wave modulated at 120 Hz....at least 
here in NA :-)  This comes as no surprise--remember, the light will 
conduct on both the positive and negative sides of the cycle.  

2)Also, I'm assuming the large majority of this light is coming from 
excited gases--He or Na lights. They are roughly square waves with poor 
HF response--I'm extrapolating this from the FFT, as I don't have a good 
way to measure a single light's waveform without clipping in the 
receiver...guess I need a gain control....you can see the first four 
harmonics (120, 240, 360, 480 Hz) are quite strong and audible, but they 
drop off rapidly after that.  I would guess that this is due to slight 
rise and fall times in the streetlights.  

I was using a Clairex CLD-142 Si photodiode without any filtering.  A 
major source of the signal was likely the 850nm peak of excited sodium, 
since that falls right in the heart of the detectors sensitivity.

Anyway, the site is http://mail.rochester.edu/~af006m/index.html and 
follow the "radio" link...475 THz is still RF to me :-)  Enjoy!

Andy, K0SM/2