From: SFPhysics@aol.com
Date: Mon Apr 01 2002 - 00:56:53 PST
From: SFPhysics@aol.com Message-ID: <12e.f0f91df.29d97ad5@aol.com> Date: Mon, 1 Apr 2002 03:56:53 EST Subject: Modulating a laser beam for sound transmission...
<<
>At a past TI workshop we were shown how to wire a
>flashlight or laser so that it could be hooked up to a
>radio. The beam would then travel to a photo cell that
>was hooked up to an amplifier or speaker. You could
>hear the music from the speaker. Amazing! I have
>lost my notes and would love to build this. Can
>anyone respond and talk me through the construction
>with some clear instructions. Its a very dramatic
>demo and other teachers would probably enjoy this as
>well. Thanks, Jeff in Vacaville
>>
Greetings Jeff:
Since no one has answered your plea let me try to help. If you take apart an
old cheap radio with a small speaker so that the speaker has no grille in
front of it, you can attach a small mirror with glue or better yet
double-sided foam sticky tape so that the mirror sits between the center
where the voice coil is and the solid rim. When the speaker delivers the
sound the mirror will move with the flexing speaker cone. A light or laser
beam pointed at the mirror will then have a moving reflection that is the
modulated sound. To pick up the sound all one needs is a silicon photo cell
with a cable to plug into a small amplifier. The reflected beam is best
centered on the edge of the silicon cell so that movement of the beam creates
varying electrical output by the cell as the beam spot moves more onto or off
of the cell's surface. The small electrical variances are then easily
amplified by even the cheapest amplifier. The only real necessity of the
amplifier is that it be a capacitor input (high impedance) so the current
from the cell will not be shorted out. Radio Shack should have all of the
things you need though their prices are astronomical in comparison to most
catalogue sales; but, they are handy.
For short distances you can get by with aluminum foil in place of the mirror
on the speaker with the advantage that the lesser mass makes for lesser high
frequency sound attenuation (this thing won't be HiFi no matter what you do).
For greater distances a good collimated HeNe laser, a first surface mirror,
a focusing lens, and twin silicon cells hooked positive to negative then fed
to the amplifier can get you a transmitted signal for up to hundreds of yards.
Another advantage is that you can check out your TV remote control with the
silicon cell connected to the amplifier. A remote held in front of the cell
will make the amplifier speaker emit a buzzing sound from the digital pulse
modulation of the IR beam when you press any button. Neat huh?
Lastly, just so this is not thought of as something new, Thomas Edison and
others were transmitting sound on light beams before 1900! They even used
amplitude modulation (AM as in AM Radio) by sending the light beam through
Kerr Cells which varied the light output according to the voltage applied to
the cell. Pretty smart those scientists of 100+ years ago.
Best wishes to all on the list,
Al Sefl
PS: Jeff if your email will take JPEG attachments I will draw you the
schematics and get the part numbers for the Radio Shack stuff. The whole
thing can be done for under $25. Guessing - AM Radio $8, Amplifier/Speaker
$12, silicon cell $3, mirror 35 cents, other misc $2... Opps, forgot the $5
laser pen... Make that $30 total...
I haven't seen the Snack version of this but this is what I started with in
1966 and it worked well though I used flashlights instead of lasers which
were thousands of dollars back then. Now that I think of it I really feel
old because the amplifiers of that day commonly had thermionic emmission
devices (Edison Effect) that were called vacuum tubes. Physics classes of
today don't even discuss the Edison Effect, do they?
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