Re: Optical Communication

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

Congratulations on your set-up to transmit sound over a modulated
beam of an LED.  This is good for short distances but it is just as
easy to substitute a Metrologic laser pointer for the LED so
the sound can be transmitted several thousand feet.

For further details send me a stamped self-addressed envelope
My address is listed in the AAPT Directory of Members.

Herb Gottlieb

On Thu, 05 Apr 2001 14:58:38 -0500 Kossom <MKossover@NEWMAN.K12.LA.US>
writes:
> Howdy-
>
> A couple of weeks ago, I attended the NSTA convention in St. Louis.
> The
> folks from the Exploritorium had many cool demos. One of the demos
> used a
> modulated LED and a solar cell connected to amplified speaker to
> send sound
> information.
>
> I wired up the system with a PASCO SF-9582 AC/DC 12V max power
> supply in
> series with a 100 ohm resistor to an ultra bright 18000 mcd LED.
> They claim
> the LED is orange. It looks red to me, but I'm colorblind. Taking
> the output
> from the headphone cable, I connected it in parallel with the LED.
> This
> arrangement modulates the LED to the frequency of the sound
> (electrical
> impulses).
>
> It is a small solar cell. It comes with no leads, as the top and
> bottom are
> the opposite poles. Rather than soldering on leads, I used alligator
> clips
> with tape insulating them on one side or the other as appropriate.
> The other
> side of the alligator leads I connect to US$ 19.99 (on sale) set of
> amplified speakers.
>
> When I brought the LED near the solar cell, only very faint music
> could be
> heard. I thought that perhaps the LED wasn't bright enough. By
> accident, I
> turned the LED away from the cell and the sound continued. I noticed
> that
> the sound continued while the wires were close together. I presume
> that the
> normal parallel or coax wire arrangement masks the external magnetic
> field
> produced by the wires carrying the sound information, but when I
> converted
> them to single wires, the changing magnetic field of headphone wire
> induced
> current in the speaker's wire.
>
> Eventually, I realized that I had connected the alligator clips
> incorrectly
> to the mini-plug on the amplified speaker. Once I corrected that, I
> was able
> to get a fair bit of sound out of the speaker,  but it was marred by
> a loud
> humming. I recognized it as AC hum. Switching to a 6V lantern
> battery
> eliminated the hum. Apparently, even when using the DC side of the
> PASCO
> power supply, there was a fair bit of power fluctuation.
>
> Another interesting effect is that the solar cell can be brought
> near a
> monitor. The monitor's flashing creates a distinctive buzz on the
> speaker.
> Different refresh rates create different pitch hums.
>
> Marc Kossover
> mkossover@newman.k12.la.us


Herb Gottlieb from New York City
(Where it's nice to live but I wouldn't
want to be a tourist here)
herbgottlieb@juno.com