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.