(Optical communication theory of operation)
I did notice that you can very slightly see the brightness fluctuate.
And I was thinking about data communication with this,
Thank you for the Info
You're welcome.
For fun sometime, I suggest connecting a scope across your
solar cell to see the resulting waveform. With another
scope channel, have a look at the original waveform.
You will probably notice some significant waveform distortion
as the signal works its way through the laser and your solar cell.
Both of these parts are "non-linear" and will *not* give you a
low - distortion waveform at the receiving end until you re-center
the zero-crossing point of your modulator and adjust the current
level to stay away from over - and under - modulation (driving the
laser too much or too little with the sound signal). After you do
that, both waveforms will look similar.
The laser is controlled by a 'constant current driver' circuit, meant
to prevent the laser from overheating when using fresh batteries and
to keep laser emission constant as the batteries flatten out.
Each alternate half cycle, you are subtracting from available battery
current ("bucking" the current). Every other half-cycle of your
modulation signal, you are increasing the current available to the
laser ("boosting" the current). This is largely what is causing
the laser brightness to be "modulated" or controlled by your sound
source.
Meanwhile, the 'driver' circuit is doing its best to even out those
current changes! Ironically, the poor (high-frequency handling)
characteristics of the 'driver' circuit in your laser pointer is the
reason that you can modulate the laser beam at all.
The "buck" and "boost" terms are industry standard descriptions
for switchmode power supply design. Now you know that a "boost"
converter places two power sources in series and in-phase to increase
output voltage and a "buck" converter puts these series power sources
180 degrees out of phase to reduce output voltage. Very efficient.
What if you used two lasers at one end and two photodiodes at
the other end, arranging one laser to become brighter during positive
transitions of the source sound and the other laser to be brighter
during negative transitions of the source sound? Put the photo-
diodes in series and connect your amplifer at the center tap.
Now your system is much more immune to interference from flashbulbs
and reflections, because these 'noise' sources cause both photo-
diodes to conduct more readily, 'phase-canceling' the interference.
Is this stuff fun, or what?
--Winston
