Given a VEX Cortex with a motor and potentiometer, this program will allow you to control a pointer in order to spell arbitrary strings.

simpleOuijaController.c

#pragma config(Sensor, in1,    POT,            sensorPotentiometer)
#pragma config(Motor,  port1,           MOTOR1,        tmotorVex269_HBridge, openLoop)
//*!!Code automatically generated by 'ROBOTC' configuration wizard               !!*//

const static int TOLERANCE = 1; // Leeway for pointing to letter
const static int MOTOR_SPEED = 32; // Speed at which to move motor
const static int WAIT_TIME_SEC = 2; // Number of seconds to wait on each letter

// Function should return degree value of position of letters on the dial.
// (Forgive me, for I know not of how to implement a proper lookup table in C)
// switch statement would also work here
int findPosition(char c){
    if(c == 'a')
	      return 0;
    else if(c == 'c')
	      return 10;
    else if(c == 'k')
        return 20;
    else if(c == 'r')
        return 30;
    else if(c == 's')
        return 40;
    else if(c == 't')
        return 50;
    else
	      return 0;
}

int getDirection(int cur, int next){
        if(cur > next)
           return -1; // go Backwards
        else
           return 1; // go Forwards
}

int queryPot() {
	return SensorValue(POT);
}

bool withinBounds(int pos){
	return pos < (queryPot() + TOLERANCE) && pos > (queryPot() - TOLERANCE);
}

void moveMotor(int speed, int direction){
	setMotor(MOTOR1, (speed * direction));
}

void pointToCharacter(char c){
	int dir = getDirection(queryPot(), findPosition(c));

	while(!withinBounds(findPosition(c)))
		moveMotor(MOTOR_SPEED, dir); // Move motor until we are within the tolerable bounds of the letter.

	moveMotor(0, 1); // Stop motor on letter
}

task main() {
  // You could iterate over a string here.
	pointToCharacter('c');
  wait(WAIT_TIME_SEC); 
	pointToCharacter('a');
  wait(WAIT_TIME_SEC); 
	pointToCharacter('t');

  // Return to home position
  wait(2*WAIT_TIME_SEC);
  pointToCharacter('a');
}