[Laser] First test of sending data via fluorescent or High Pressure Sodium lamps

[Kerry Banke]

Over the holiday weekend I completed a first pass at an experiment I 
have been wanting to perform for several years.  The experiment was 
to attempt to send data via a fluorescent lamp or HPS lamp by 
changing the frequency used to power the lamp.  The setup right now 
consists of an Elgar 1Kw variable frequency AC  power supply which 
was kindly supplied  on loan by  Greg, K6QPV. This unit has a manual 
frequency control but also an external DC  analog frequency control 
input.  I basically made an interface to take the audio output from a 
laptop computer, square it up, divide the frequency by two (the lamp 
modulation is at the second harmonic) and applied that to a phase 
detector to phase lock the power supply to the audio signal frequency 
divided by 2.  I fed the phase detector output to the dc frequency 
control of the power supply using it as a VCO in the phase lock 
loop.  I currently amp using an audio frequency range of 125-135 Hz 
which puts the power to the lamp in the 62.5-67.5 Hz range which the 
lamps don't seem to mind.  The software I tried so far was Andy's 
(K0SM) Laserscatter and the sequential MT Hell found in the Spectrum 
Lab software.  These were chosen as they both send sequential single 
tones and are nicely compatible in center frequency and frequency 
span.  Text messages were successfully sent and received  for both 
software only over short distances but with the optical path set for 
weak signal reception. Tests were run using both a 150 watt HPS lamp 
(16000 lumens) and a fixture with two 48" fluorescent bulbs.   The MT 
Hell approach is interesting but takes some getting used to as the 
decoding is by watching the characters form on the spectrum analyzer 
waterfall display.  Andy's software I think is a perfect fit for this 
application .  It generates the sequential tones and receives/decodes 
back into text automatically.  I believe the next step is to solidify 
the hardware and place it on a mountain top for testing as a 
beacon.  We will have complete control via a radio link to turn on 
the beacon only when needed to prevent unneeded light pollution.  If 
this turns out to be of interest to others, I may design a simple, 
stand alone power source to that can be readily duplicated.  Stay tuned.
73 - Kerry N6IZW - San Diego Microwave Group