关于pwm控制led灯亮度的问题

请叫我根硕

我利用红外测距传感器和一个恒流驱动模块控制led灯的明暗，为什么我的pwm电压不会变，灯一直是暗的呢？
程序如下：
int pin = 9;  //analog outint apin0 = A0;  //analog in0
int apin1 = A1;  //analog in1
int apin2 = A2;  //analog in2
int sensorvalue0=0, sensorvalue1=0, sensorvalue2=0;  //three values provided by sensors
int sensorvalue;  //the sensor which is the largest, i.e. the right sensor value to use
int outputvalue=255;  //output value for the PWM, 255 corresponds to the lowest output voltage, in order to protect the LEDs
int dpin0 = 2;  //digital out, that is to control the relay
int dpin1 = 3;
int dpin2 = 4;

void setup()  
{  
    //define each pin INPUT or OUTPUT
    pinMode(pin, OUTPUT);
    pinMode(apin0, INPUT);
    pinMode(apin1, INPUT);
    pinMode(apin1, INPUT);
    pinMode(dpin0, OUTPUT);
    pinMode(dpin1, OUTPUT);
    pinMode(dpin2, OUTPUT);
    Serial.begin(9600);
}   

void loop()  
{
  //read the values provided by the sensors
  //constrain these values between 0 and 500
  sensorvalue0 = analogRead(apin0);
  sensorvalue0 = constrain(sensorvalue0, 0, 500);
  sensorvalue1 = analogRead(apin1);
  sensorvalue1 = constrain(sensorvalue1, 0, 500);
  sensorvalue2 = analogRead(apin2);
  sensorvalue2 = constrain(sensorvalue2, 0, 500);

//select the largest value from the three
//then put the corresponding pin selected from dpin0, dpin1 and dpin2 to HIGH and put the others to LOW
if(sensorvalue0>sensorvalue1){
   if(sensorvalue2>sensorvalue0 && sensorvalue2>200){
     sensorvalue = sensorvalue2;
     digitalWrite(dpin2, HIGH);
     digitalWrite(dpin0, LOW);
     digitalWrite(dpin1, LOW);
   }
   if(sensorvalue0>=sensorvalue2 && sensorvalue0>200){
     sensorvalue = sensorvalue0;
     digitalWrite(dpin0, HIGH);
     digitalWrite(dpin1, LOW);
     digitalWrite(dpin2, LOW);
   }
}
else{
   if(sensorvalue2>sensorvalue1 && sensorvalue2>200){
     sensorvalue = sensorvalue2;
     digitalWrite(dpin2, HIGH);
     digitalWrite(dpin0, LOW);
     digitalWrite(dpin1, LOW);
   }
   if(sensorvalue1>=sensorvalue2 && sensorvalue1>200){
     sensorvalue = sensorvalue1;
     digitalWrite(dpin1, HIGH);
     digitalWrite(dpin0, LOW);
     digitalWrite(dpin2, LOW);
   }
}

//remap the input values from the sensor to the range for output(245~255)
//!!NOTE: if the number of the LEDs has changed, then the lower bound of the range needs to be changed also
//255--0ma     0--700ma
//so if the number of LEDs has increased, then you have to change the 245 to a lower number, I have written down this relation on the paper I give you  
  outputvalue = map(sensorvalue, 0, 500, 245, 255);
  outputvalue = constrain(outputvalue, 0,255);
  analogWrite(pin, outputvalue);

  //used for debug using the serial(usb) port
  //Serial.print("sensor = " );                       
  //Serial.print(sensorvalue);      
  //Serial.print("\t output = ");      
  //Serial.println(outputvalue);

  delay(2);
}  
有大神能帮我解答吗，谢了~