[Laser] Simple optical beacon experiment in San Diego

[TWOSIG at aol.com]

Kerry
 
Thank you for sharing these experiments with us.  It got me to  thinking and 
wondering.  
 
I have some questions.  If they are about something that you do not  know the 
answer to from what you have already done, please consider them  suggestions 
for further investigation.  They may not be worth the effort to  investigate, 
but I hope they stir some thought in the group.
 
Can you see the LED from your observation point with the un-aided  eye?  In 
darkness the eye will open to about 7mm and from what I understand  about 
binoculars a 7X50 will give an image area at about the same  brightness.  The 7X20 
rifle scope should give an image about 16 percent as  bright as the 
binoculars.  The LED would only be a point, so I am not sure  if the relationship makes 
sense.   Also could you see any twinkling of  the light from the LED?  If it 
is not easy to see the LED because of  surrounding light sources, you might be 
able to look through a tube to block  some of that light.
 
If I understand your description, the ID is CW, not MCW.  I would like  to 
know more about the way you set up the system.  Was it a microcontroller  
programed?  And could the control link do more than just turn it on and  off?
 
What is the focal length of the 4" glass lens on your receiver?    What are 
the dimensions of the photo detector?  What I am wondering about  is the field 
of view of the sensor and how that relates to the noise sources you  describe. 
 I have a simple 300 mm lens and one of my detectors is about 4.5  mm square. 
 If I used that at 8 miles, then it would see an area about  634 feet on a 
side.  For similar numbers on your setup, are the noise  sources in that field 
of view, or are they outside that range?  If the  latter, are you getting 
reflections of light from the ground and objects in the  field of view, maybe from 
atmospheric scattering, or maybe from reflections in  your receiver?  I was 
going to suggest that you try a 500mm or longer focal  length telescope - 114 mm 
would be about the same diameter as your 4" glass  lens - to get a narrower 
field of view, but it seems that is unlikely to  help.  Maybe an optical scope 
would have been built with more experience  eliminating stray light.  It seems 
like you have plenty of sensitivity in  the detector, just too much 
interference.
 
What are the light sources in the area?  Sodium vapor, Mercury vapor,  
incandescent, neon?  It may be easier to find a filter for the gas  discharge types 
than for incandescent sources.  There may also be a  component in those noise 
lights that relates to three phase power.  I doubt  that any single light will 
have a 180 or 360 component, but there may be lights  that group into three 
sets of phase relationship.
 
I get lots of hum from incandescent lights, which I presume is 120  Hz.  I do 
not notice any strobing from the same source, so I am puzzled  -  it might be 
that the electronic sensor is picking up only that small  fraction of the 
light from the lamp that is pulsing and my eye does not  have the dynamic range 
to see the strobe effects on top of the other light   -- or it might be that 
the hum I am hearing from the sensor is from the IR  spectrum which I cannot see.
 
Since you are driving the LED with a square wave, can you detect the  
harmonics of that?
 
Are you driving the LED at its rated current?  Since you are running  it at  
700 Hz, have you considered running it at a higher current  since you are only 
50% duty cycle?  If I tried it, I would probably ony go  150 percent, or at 
least monitor the heat sink temperature.  It might not  be safe to increase the 
drive current for the CW for your ID.
 
 
If you are considering some follow on testing, I can suggest a sine wave  
driver circuit that can be keyed on and off fairly easily.  It is a simple  
circuit that I thought up while working on my AM LED demonstration.  I only  drive 
30 milliamp, but it is linear and should scale up to what you want to  use.  
If I remember correctly the 1 watt LED runs about 350 milliamp.   This AM 
circuit, if adjusted for 250 ma and driven at 50 percent modulation  would peak at 
375 ma and then drop to 125 ma.  If you have the ability  to key different 
lines, you should be able to set the modulation percentage,  then step to another 
level, say 10, 25, 50, 75, 100, and 125 percent  modulation.
 
I hope I have given you some useful ideas.  Thank you again for  sharing your 
work.
 
 
James 
N5GUI