PROJECTS Amateur Scientist
circuit to test infrared (IR) remote controllers.
You don’t need to build the circuit in Figure C
if you have a Bluetooth speaker that includes an
audio input jack for an external signal. All you’ll
need to make an audio frequency light receiver is
a suitable detector.
I’ve used a variety of silicon photodiodes for
this purpose. A typical one is OSRAM’s BPW34.
These are miniature solar cells installed in a
miniature epoxy or metal package with a glass
window. Figure
D
shows the voltage produced by
a photodiode in late afternoon sunlight.
Figure
E
shows an Anker Bluetooth speaker
connected to a silicon solar cell. The positive lead
from the solar cell is connected to the tip of a ⁄"
phone plug inserted into the speaker’s input jack.
The negative lead goes to the ground connection
on the plug.
Figure E also shows a photodiode soldered to a
phone plug that can be inserted into the speaker’s
input jack. This works well for testing remote
controllers. If no tone is heard from the speaker
when the remote’s button is pressed, chances are
the remote’s battery needs replacing.
You can even use an LED as a light-sensitive
photodiode instead of a silicon light sensor. Use
the same type of LED for both the transmitter and
receiver. Near-IR LEDs (880nm) will work best.
This is the most common LED used in remote
controllers, so you can demonstrate reception of
a tone-modulated IR beam simply by pointing a
remote controller at the receiver’s LED.
Be sure to set the speaker’s volume to its
lowest level when experimenting. A 0.5-volt
silicon solar cell used as a light sensor can cause
a very loud sound from the speaker!
SENDING VOICE OVER A BEAM
OF SUNLIGHT
After Alexander Graham Bell invented the
telephone, he pioneered ways for beams of
sunlight to carry musical tones or even voice. He
also pioneered the development of light receivers
that converted a tone or voice superimposed onto
a beam of sunlight back into sound. He called this
new technology the photophone.
On February 19, 1980, my wife Minnie and I
visited a small parking lot in Washington, D.C.
A former building at that site was home to Bell
D
E
A silicon photodiode generates a voltage when
illuminated by light.
A light receiver can be easily made by connecting a
photodiode or solar cell to the input of a Bluetooth
speaker.
CAUTION: Be careful if you’re
connecting a solar panel that has multiple
cells or an output exceeding a few volts,
for the high voltage might damage your
Bluetooth speaker. If you try this, insert
a 0.1µF capacitor between the positive
solar panel connection and the speaker’s
positive input, and use the panel indoors,
or outdoors only at night.
86 makezine.com
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