Sound Modulated Light from Light Emitting Diode.
This project can amplitude modulate the light being emitted from
an LED using the 8 ohm earphone output from a small radio or amplifier.
For demonstrating, it is very convenient to use a small radio
to supply the audio signal. A small hand held audio amplifier
from Radio Shack can be used to drive the LED if you are using
The receiver is simply a photocell connected to the input of a
tiny hand held audio amplifier from Radio Shack. Since the light
intensity is varying according to the original modulating audio signal,
the photocell will put out an audio voltage that is like the original
signal and will be heard in the pickup amplifier.
The range of transmission can be greatly increased by:
- Putting a convex lens in front of the LED and focusing the light
into a narrow beam.
- Putting a lens in front of the photocell and focusing
(concentrating) the light beam on to it.
- Using a Super Bright LED; the kind that say something like
2000 mc at 30 ma. Most LED's put out typically 300 mc.
- All of the above will give the maximum range and performance
but the focusing with the lenses requires careful adjustment.
/ Switch. .---------------.
.-------/ o--------. | ooo |
| | | ooooo |
--- + / | ooooo |
- \ 1 K ohm | ooo |
--- 9V Battery / | |
- \ | Audio |
--- / | Amp |
- - 10 uf | ---------------
| cap | | |
| | |+ | Audio modulated | |
| }--------| |--| light from LED. .----. | |
| 8 ohm | | V - - - - - - -} | |---- |
| audio sig. --- LED | |------ '
| input. | '----'
| }-------------| Photo cell
It is very fascinating to carry around just the photocell and
the small hand held amp and listen to various light sources.
We put the photocell in front of a headlight of a parked truck
with the engine idling. We could hear noises from the engine.
We even were able to hear an occasional buzz as a bugs would
fly through the headlight beam. Some flashlights will make a
"ding" sound if they are tapped while shining on the photocell.
If a fast rotating shaft is marked with a black marker, and the
photocell is placed near it, a tone will be heard in the amp and
the rpm can be determined by comparing the musical pitch with a
pitch chart. If you don't have a chart you can determine a pitch
frequency by starting at A 440 hz and multiplying by 1.059463 each
time to go up the chromatic scale or dividing by the same number each
time to go down the chromatic scale. To check if you are doing it
right, you should arrive at 880 hz after multiplying 12 times. You
can go down the scale by dividing by the same number above. After
12 divisions, you should arrive at 220 hz. A is the only note of
the scale that has a good even number as a frequency value. Each
octave is exactly half or double a given frequency.
Anything that emmits light can be a possibility for discovering