[Laser] RTTY over light, anyone tried it?

[n9jim-6 at pacbell.net]

Another relatively easy thing to try is simply driving the tx with a serial port. And rx adding a comparator.  Works for relatively strong signals.

Jim
N9jim


James Whitfield wrote:
> Paolo
> Since you are using an LED for your emitter, perhaps you would consider not 
> trying to "square" the drive signal.  The following is based on the circuit 
> that I use in the transmitter of my "Talk on a Beam of Light" demonstration.
> Assume for the moment that you are going to use a PC sound card system to 
> generate signals, even if you intend to transfer them to an MP3 player for 
> testing in the field or a beacon.  Take that audio signal and run it through 
> a simple op amp stage with variable gain then to an op amp LED driver stage. 
> It would take a dual op amp operating with a single 12 volt supply, and use 
> a pseudo ground at 6 volts.  The first stage would be capacitively coupled, 
> or transformer coupled, to the audio source.  At zero input, the first stage 
> output rests at 6 volts.  As audio is fed into the system, adjust the gain 
> so that the audio peaks swing within the linear range of the particular op 
> amp, perhaps 2 volts on the negative swing and 10 volts on the positive. 
> Later you can always boost the gain so that the squaring function becomes 
> necessary.
> Run the output of the first stage to a resistor divider, the center of which 
> is connected to the non-inverting input to the second stage.  The second 
> stage output would go to the base of an NPN transistor.  The LED is wired 
> from the V+ supply to the collector of the transistor.  The emitter of the 
> transistor goes through a sensing resistor to ground.  There is a line from 
> the emitter to the inverting input of the second stage.
> At no signal, the first stage output is 6 volts, passes through the resistor 
> divided to set the stage two input to 3 volts.  It will drive the base of 
> the transistor to draw current through the LED and down to the emitter, then 
> through the sense resistor.  Select the value for the sense resistor so that 
> at 3 volts the current flow will be equal to the rated continuouse current 
> of the LED, or less if you choose.  When a signal is supplied, the current 
> through the LED will be proportional to the voltage out of the first stage. 
> Note that as long as you do not over drive the op amp, the average current, 
> and hence the heating of the LED, will remain constant.  A symetrical 
> waveform will provide equal drive above the average and below on each cycle, 
> which simplifies the adjustment to protect the LED from overdrive.
> Perhaps you would be able to run comparison of the different digital methods 
> when used on an optical link, PSKxx versus RTTY ( FSK ) versus MFSK.   There 
> may also be value in comparing different audio frequency "carriers", say 
> 250, 500, 750, 1000, 1250, 1500 Hz for PSKxx or the other modes.  You may 
> find benefits for one mode or frequency range for a given data rate.
> Sending an continuous valued rather than squared light signal may allow you 
> to try something like MT63 without having to resort to pulse frequency 
> modulation or pulse width modulation.
> Please keep us posted on how your work is going.
> One last thought, had you looked into Laser Scatter by K0SM as a digital 
> mode?  I think of the data rate as being too slow for keyboard conversation. 
> It might suit your needs.
> James
>  n5gui
> ----- Original Message ----- 
> From: "Paolo Cravero" <pcravero at as2594.net>
>> So far I have built an MCW transmitter and an OPT202 receiver. The TX is a 
>> 10mm red LED, and soon the RX will have a nice telescope mount so that 
>> first "beaconed" outdoor tests can take place. No-lens tests took part 
>> indoor on 12m paths, both LOS and NLOS.
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