From: SFPhysics@aol.com
Date: Wed Apr 03 2002 - 15:17:11 PST
From: SFPhysics@aol.com Message-ID: <d7.1582ee8f.29dce777@aol.com> Date: Wed, 3 Apr 2002 18:17:11 EST Subject: Snack Book Modulated LED pt. 2
Dear Dr. Paul and Listers:
Thanks Paul for the heads up on the web address of the Modulated LED from the
Square Wheels Snack Book. The setup is simplicity in itself with the small
radio providing the audio modulation to the 9 Volt battery powered LED. This
demonstration should also be good to show the Inverse Square Law as the sound
output should be tied to the distance the solar cell is from the LED source.
The distance for the light to carry the AM audio information should thus be
somewhat short.
Pam, I will send you a JPEG on how to use two solar cells and a cardboard
tube fitted with an aluminized balloon diaphragm so you can send messages for
many meters. A rubber balloon stretched over the tube with a small mirror
stuck to it actually works better for longer distances. No amplifier is used
for transmission but you will need to steady the tube on a solid mounting as
a laser is pointed on the diaphragm for reflection to the twin photocells.
As you talk into the cardboard tube the diaphragm moves the laser beam
between the two cells and you can send your voice over a light beam. My
students achieved quite a distance with this. Using reflected sunlight they
managed to transmit their voices from the park across the street. In 1892
Edison called this thing a Photophone! In those days before the De Forest
Audion (vacuum tube amplifier 1906) Edison used his invention the
Microtasimeter, a Lalande Cell for power, and a telephone receiver to receive
sound on a light beam. The Microtasimeter resistance to current was
extremely sensitive to heat variations so the reflected sunlight modulated by
the voice of the sender was heard in the old style magnetic telephone
receiver. One hundred and ten years ago!
Best wishes to all,
Al Sefl
This archive was generated by hypermail 2.1.3 : Mon Aug 05 2002 - 09:21:41 PDT