#define FREQUENCY 2
//#define FEEDBACK_TARGET 512

short period = 1000 / FREQUENCY;
#ifdef FEEDBACK_TARGET
byte power = 0;
#else
byte power = 255;
#endif

// x must be in degrees between 0 and 180
float fastSin(float x) {
  x *= 180 - x;
  float sine = 4 * x;
  sine /= 40500 - x;
  return sine;
}

void setup() {
  digitalWrite(9, LOW);
  digitalWrite(10, LOW);
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
}

void loop() {
  byte a = 0, b = 0; // PWM outputs
  short angle = (millis() % period) * 360 / period;

#ifdef FEEDBACK_TARGET
  short feedback = analogRead(A0);
  if (feedback < FEEDBACK_TARGET && power < 255) {
    power++;
  } else if (feedback > FEEDBACK_TARGET && power > 0) {
    power--;
  }
#endif

  if (angle < 180) {
    a = fastSin(angle) * power;
  } else {
    angle -= 180;
    b = fastSin(angle) * power;
  }

  analogWrite(9, a);
  analogWrite(10, b);
}