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My First Photophone

February 20, 2011

The first prototype!  Success.

The signal is pretty granular, but it is still a signal:

I’m mapping a pretty narrow range of values to the height of the screen.

For a while now I’ve been interested in the Photophone–the first wireless telephone–invented jointly by Alexander Graham Bell and Charles Sumner Tainter on February 19, 1880 (see https://mfleisig.wordpress.com/2011/02/09/photophone-bagpipes/).  They reflected sunlight off a flexible mirror, vibrated by the human voice through a mouth pipe, and projected it on a photocell several hundred meters away, producing an electric current which was converted back into sound.  It seemed like it was high time to see what would be involved in getting a signal with the photoacoustic effect.

The site Photophones Revisited (http://www.bluehaze.com.au/modlight/GrothArticle1.htm), has a lot of good material, but I really wanted to get a fast prototype done today and just see if I could get any signal at all, so you’ll see that I am doing without the niceties of lenses and mirrors that would really help to fine-tune it.  One point Mike Groth makes that I found very helpful is that the flux from a red LED is nearly as high as a bright white one, and sufficient for demonstrations and prototyping.  I confirmed this with some quick tests of different LEDs:

Sure enough, in lighting conditions where I’m getting readings of 160 (0-255) at the top of the tube (facing down), I got 180 for the green LED, 238 for the round red one, 245 for the rectangular red one, and 250 for the super bright white one.  I worked with the round red one as it was a little easier to manipulate.  If I reproduce this experiment, I’ll probably try it with the super bright white one next time.

My basic plan was as follows:

Shield the photocell and LED from each other, and from other light sources as well by placing them inside a tube.  Aim the LED away from the photocell, and at a diaphragm vibrated by sound waves from my voice through another tube nested inside.

The first thing I did was rig up a little reflector to put between the LED and photocell, and to support the LED:

I covered it in foil to prevent light from getting through the spoon.  Then I wrapped the end of a flexible tube in foil and fastened the two together:

Then I attached the LED, trying to be careful not to ground out the wires on the foil:

The LED, sound tube and reflector are on the right, and the photocell is on the left:

I wrapped the whole assembly in foil again to prevent any stray light from getting in on the right side.

The code is straightforward, and modified from Tom Igoe’s sketches at Arduino.cc (http://arduino.cc/en/Reference/Serial) and Processing.org (http://processing.org/reference/libraries/serial/index.html).

Here’s the Arduino code:

int analogPin = 0;
int analogValue = 0; // integer to print

void setup() {
// open serial communications at 9600 bps
Serial.begin(9600);
}

void loop() {
// read the analog input, divide by 4:
analogValue = analogRead(analogPin) /4;

// print:
Serial.print(analogValue, BYTE); // Print the raw binary value analogValue
Serial.println(); // print a linefeed and carriage return

Here’s the Processing code:
import processing.serial.*;

Serial myPort;
int graphXPos = 1;

void setup ()
{
size(400, 300);
println(Serial.list());
myPort = new Serial(this, Serial.list() [0], 9600);
myPort.clear();
background(48,31,65);
}

void draw ()
{
}

void serialEvent (Serial myPort)
{
int inByte = myPort.read();
);
stroke(123,128,158);
if((inByte != 10) && (inByte != 13)) {
float y = map(inByte,140,150,0,height);
println(inByte);
line(graphXPos, height, graphXPos, height-y);
}
if (graphXPos >= width)
{
graphXPos = 0;
background(48,31,65);
}
else {
graphXPos++;
}
}

delay(10);
}

The signal is quite rough, and the mapping has to be changed frequently, but it is a good start:

It’s not as sensitive to sound as it is to my breathing in and out, but I have built a simple photoacoustic microphone.  The next step will be to improve the signal with a stronger light source, mirrors, lenses, and some of the other techniques that Mike Groth outlined (Photophones Revisited [http://www.bluehaze.com.au/modlight/GrothArticle1.htm]), specifically, using Vibrating Grid Modulators, and possibly trying to polarize the light, if I can find a way to do it without killing myself.  Regardless, if I focus on a signal processing system that does not depend on a very fine resolution, I can focus on developing the capture, output and storage sides of this project.

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