I saw a Monty Python skit that used semaphore. It was interesting so I decided to built a robot semaphore system.

I bought two smaller steppers from recycledgoods.com. They are pretty low power so I was able to power them from USB. I used an LCD from fungizmos.com to show the letter that is being flagged.

The circuit to drive the stepper and lcd is pretty standard. There is a good stepper tutorial on the arduino site. Fungizmo has the circuit for the LCD.

My arduino code basically reads a number from serial input. The number must be in the range from 00 to 26. With 00 = space, 01 = A and so on. Once it reads the number, the arduino then drives the steppers to flag the letter. I wrote a little perl program that will read from standard input, then send the characters over serial to the arduino, one character at a time. That way I can just cat | any text file into the arduino.

My arduino code:

#include 
#include 

int lcd_addr = 0x50;		//default I2C hex address from datasheet

// change this to the number of steps on your motor
// My stepper is 7.5 degrees per step, or 48 steps per revolution
#define STEPS 48

// create an instance of the stepper class, specifying
// the number of steps of the motor and the pins it's
// attached to

Stepper leftFlag = Stepper (STEPS, 8, 10, 9, 11);
Stepper rightFlag = Stepper (STEPS, 7, 5, 6, 4);

// Set stepper starting position
int leftFlagStep = 48;
int rightFlagStep = 0;

int incomingByte1 = 0;		// for incoming serial data
int incomingByte2 = 0;		// for incoming serial data

int debug = 0;

// LCD buffer
char lcd[20] = "Ready          ";

// Letter array. we read a number over serial, then convert it to a letter 
char letters[] =
  { ' ', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N',
  'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'
};

//  Create and array that has the flag positions.  
// 395 = 54
// 360 = 48
// 315 = 42 
// 270 = 36 
// 225 = 30 
// 180 = 24 
// 135 = 18
//                           0   1   2   3   4   5   6   7   8   9  10  11  12  13 
 14  15  16  17  18  29  20  21  22  23  24  25  26 
//                          '',  A , B , C , D , E , F , G , H , I , J , K , L , M ,
 N , O , P , Q , R , S , T , U , V , W , X , Y , Z 
int leftFlagPositions[] =
  { 48, 48, 48, 48, 48, 29, 36, 42, 54, 54, 36, 24, 30, 36, 42, 18, 24, 30,
  36, 42, 24, 30, 42, 36, 42, 36, 36
};

// 225= 30
// 180= 24
// 135= 18
// 90 = 12
// 45 =  6
//  0 =  0
int rightFlagPositions[] =
  { 0, 6, 12, 18, 24, 0, 0, 0, 12, 18, 24,
 6, 6, 6, 6, 12, 12, 12, 12, 12, 18,
  18, 24, 30, 30, 18, -6
};

void
setup ()
{

  delay (1000);			//allow lcd to wake up.

  Wire.begin ();		//initialize Wire library

  // Wire library expects 7-bit value for address and shifts left appending 0 or 1 for read/write
  // Lets adjust our address to match what Wire is expecting (shift it right one bit)
  lcd_addr = lcd_addr >> 1;

  //Send lcd clear command
  Wire.beginTransmission (lcd_addr);
  Wire.send (0xFE);		//Cmd char
  Wire.send (0x51);		//Home and clear
  Wire.send (lcd);              // Put up the default message

  Wire.endTransmission ();

  // set the speed of the motor to 30 RPMs
  leftFlag.setSpeed (30);
  rightFlag.setSpeed (30);

  Serial.begin (19200);		// opens serial port, set data rate 
}

void
loop ()
{
  // check for incoming data 
  if (Serial.available () > 0)
    {
      if (Serial.available () == 2)
	{
	  // read the incoming 2 bytes then convert them to an integer
	  // convert from ascii
	  // This number will correspond to the letter to semaphore. A = 01, B = 02 and so on
	  incomingByte2 = Serial.read ();
	  incomingByte1 = Serial.read ();
	  setFlags ((incomingByte1 - 48) + 10 * (incomingByte2 - 48));
	}
      else
	{

	}
    }
}


// Move the flags to make the letter
void
setFlags (int letter)
{

  lcdPrint (letter);

  if (debug)
    {
      Serial.print ("letter = ");
      Serial.println (letter, DEC);
    }

  do
    {
      if (leftFlagStep > leftFlagPositions[letter])
	{
	  leftFlag.step (-1);
	  leftFlagStep--;
	}

      if (rightFlagStep > rightFlagPositions[letter])
	{
	  rightFlag.step (-1);
	  rightFlagStep--;
	}

      if (leftFlagStep < leftFlagPositions[letter])
	{
	  leftFlag.step (1);
	  leftFlagStep++;
	}

      if (rightFlagStep < rightFlagPositions[letter])
	{
	  rightFlag.step (1);
	  rightFlagStep++;
	}
    }
  // Loop as long as one of the flags needs to move
  while (leftFlagPositions[letter] != leftFlagStep
	 || rightFlagPositions[letter] != rightFlagStep);

  // Send out a "Z" to signal we have finished this letter
  Serial.println ("Z");
}

void
lcdPrint (int letter)
{

  // Shift everything in the LCD buffer to the left
  // Then print it to the LCD screen
  for (int cnt = 0; cnt < 16; cnt++)
    {
      lcd[cnt] = lcd[cnt + 1];
    }
  lcd[15] = letters[letter];

  Wire.beginTransmission (lcd_addr);
  Wire.send (0xFE);		//Cmd char
  Wire.send (0x46);		//Home and clear
  Wire.endTransmission ();


  Wire.beginTransmission (lcd_addr);
  Wire.send (lcd);
  Wire.endTransmission ();
}

The perl code is pretty straightforward. It reads from standard input, converts the characters to numbers, then writes the numbers to the arduino over the serial port. The syntax is: cat rickroll.txt | perl ./semaphore.pl

Here is my perl code:

use Device::SerialPort;

# Set up the serial port
# 19200, 81N on the USB ftdi driver
my $port = Device::SerialPort->new("/dev/ttyUSB0");
$port->databits(8);
$port->baudrate(19200);
$port->parity("none");
$port->stopbits(1);

# hash of letters
# send the letter number to the arduino
%letter = (
    ' ' => '00',
    'a' => '01',
    'b' => '02',
    'c' => '03',
    'd' => '04',
    'e' => '05',
    'f' => '06',
    'g' => '07',
    'h' => '08',
    'i' => '09',
    'j' => '10',
    'k' => '11',
    'l' => '12',
    'm' => '13',
    'n' => '14',
    'o' => '15',
    'p' => '16',
    'q' => '17',
    'r' => '18',
    's' => '19',
    't' => '20',
    'u' => '21',
    'v' => '22',
    'w' => '23',
    'x' => '24',
    'y' => '25',
    'z' => '26',
);


#  Read the characters from standard in
while () {
    $_ =~ tr/A-Z/a-z/;
    $_ =~ s/[^a-z ]+//gi;
    @chars = split( //, $_ );

    foreach $character (@chars) {
        print $character;
        print "Sending " . $letter{$character} . "\n";

        $port->write( $letter{$character} );

        # Poll to see if any data is coming in
        # Wait for the arduino to tell us it is finished with the last letter
        my $response = '';
        while ( !$response ) {
            $response = $port->lookfor();
        }

        # just wait for a second 
        sleep(1);
    }
}