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FIRST TEST OF SENDING DATA VIA FLUORESCENT OR HIGH PRESSURE SODIUM LAMPS

From: Kerry Banke, N6IZW
San Diego Microwave Group
kbanke (at) qualcomm.com

Date: September 5, 2006


Thought you might get a kick out of my latest weird science project. :)   Maybe a better title would be, "What weird things does a Qualcomm R&D engineer do for relaxation?"

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 digital data via a fluorescent lamp or a High Pressure Sodium (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 loaned 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 then applied that signal to a phase detector to phase lock the power supply to the audio signal frequency divided by two.  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 am 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 tubes.

  The MT Hell approach is interesting but takes some getting used to as the decoding is by watching the characters form on a 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 mountaintop 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 project turns out to be of interest to others, I may design a simple, stand alone power source that can be readily duplicated.  Stay tuned.

From: Kerry Banke, N6IZW
San Diego Microwave Group
kbanke (at) qualcomm.com

Posted by Steve Blodgett
Earthsignals.com