audio over optical links as an add on to radio stations

Samudra Haque samudra.haque at
Wed Oct 12 00:31:34 CDT 2005

Hi AMRAD folks,
What do you think of designing a circuit similiar to the last schematic
published on, I quote from
the web page:

So, what is it?

""It is a modulated light sender! Connect the input to an audio source or
microphone (a speaker will work) and the audio will amplitude modulate the
light intensity. The inefficiency of the class-A works in our favor now,
lighting the lamp to mid-brightness with no audio present. With a sensitive
detector like a phototransistor, this communicator will work several hundred
feet (at night). Best range is realized if the bulb is mounted in a typical
flashlight reflector and the detector is similarly mounted. The input
capacitor is reduced to .01 uF to give the amplifier a high-pass character
to compensate for the slow response of the bulb. The audio will sound a bit
muffled, anyway. The clever designer could use this amplifier for the
receiver, too, switching the speaker to the input for transmitting and to
the output for listening. If you choose a detector with good infrared
response, like a pin photo diode, you can add plastic IR filters to block
out ambient light and make the communicator harder to see at night.

Increasing the voltage to 12 VDC, replacing the bulb with a 3 watt, 16 ohm
speaker and replacing the .01uF with a 1uF gives an audio amp that will
deliver nearly 1 watt of audio power. The speaker will get warm, however!
(Due to the nearly 2 watts of DC power in the speaker coil.)""

I was wondering if it would be useful to build a circuit that would be
portable enough to use in a apartment or a large room or in fact put on top
of a tower/mast in a community and then fed audio from a radio/telephone
patch which would albeit slowly rebroadcast the audio in a narrow band -
without the use of actual AF or any kind of RF transmission. The
applications might be as simple as keeping the SO or XYL happy by not
rebroadcasting sounds at night, while you are trying to monitor the band -
and not using an FM transmitter to do it.

Other serious applications might be to setup a silent audio link (two-way)
between locations to connect camps together such as Burke Lake field days
with directional optical transmitter tubes (sort of point to point low power
optical links, ala lasers).

-samudra, N3RDX/S21X
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