int speakerPin = 9;
int controlPin = 0;

void setup() {
    pinMode(speakerPin, OUTPUT);
    }

/* 

play a tone with period given in microseconds for a duration 
given in milliseconds

*/
void playTone(int period, int duration) {
    // elapsed time in microseconds, the long is needed to get
    // a big enough range.

    long elapsedTime = 0;

    // note we are making a slight error here if period is not even
    int  halfPeriod = period / 2;

    while (elapsedTime < duration * 1000L) {
        // generate a square wave of the given period.  Half the 
        // period is on, the other half is off.
        digitalWrite(speakerPin, HIGH);
        delayMicroseconds(halfPeriod);

        digitalWrite(speakerPin, LOW);
        delayMicroseconds(halfPeriod);

        // count the time we just consumed for one cycle of the tone
        elapsedTime = elapsedTime + period;
        }

    }

void loop() {
    int period;

    period = analogRead(controlPin);
    playTone(period, 500);
    }