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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  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 (, 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 ( and (

Here’s the Arduino code:

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

void setup() {
// open serial communications at 9600 bps

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);
myPort = new Serial(this, Serial.list() [0], 9600);

void draw ()

void serialEvent (Serial myPort)
int inByte =;
if((inByte != 10) && (inByte != 13)) {
float y = map(inByte,140,150,0,height);
line(graphXPos, height, graphXPos, height-y);
if (graphXPos >= width)
graphXPos = 0;
else {


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 []), 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.


From → Thesis

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