[Laser] Laser comm stuff

[Andrew T. Flowers, K0SM]

Dave,

Great to hear someone else is working on this.  I've discovered that 488 
Hz is a pretty bad choice here in the US--too close to the 4th harmonic 
of 120 Hz.  I ordered some 6 MHz xtals that should move the frequency to 
around 730 Hz or so.  As far as the power ouput is concerned, I found 
that my laser pen that normally runs on 2 AAs seems happy with 5V.  I 
ran it there laser night for the better part of an hour (900 Hz/50% duty 
cycle).  It seems to be okay :-)

You might consider using a lower noise device for your preamp.  G0MRF is 
using the NE5534 (5532 for dual) device, which should be considerably 
quieter than the TL082, and is available in a DIP.  If you used a socket 
you could probably just switch them.  With my PGP/LM386 combo (yes, I 
need to get a buffer in there), I can sometimes hear my cloud 
reflections with a 4" lens.  The air can be rath hazy here this time of 
year, so it's hard to tell what's coming from what.

Andy K0SM/2
wa4qal@ix.netcom.com wrote:

>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
>
>