Reading message failed in rosserial

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I have used the code and circuit given below to glow the LED when the value from analog sensor exceeds the threshold value. and i received the error as "[WARN] [WallTime: 1422707498.360818] Serial Port read returned short (expected 77 bytes, received 19 instead) " and some times LED is glowing when the value from sensor is less than threshold value.

I use: Operating system : Ubuntu 14.04, ROS indigo, Analog sensor : potentiometer, Board : Arduino UNO.

#include <ros.h>
#include <ros/time.h>
#include <sensor_msgs/Range.h>
#include <std_msgs/Empty.h>

ros::NodeHandle  nh;

  void messageCb( const sensor_msgs::Range& range_msg)
  {
   if (range_msg.range >= 700) {                    
  digitalWrite(13, HIGH-digitalRead(13)); 
   }

  else {
   digitalWrite(13, LOW-digitalRead(13));
   }
  }
  ros::Subscriber<sensor_msgs::Range> sub("range_data", &messageCb );


  sensor_msgs::Range range_msg;
 ros::Publisher pub_range( "range_data", &range_msg);

 const int analog_pin = 0;
  unsigned long range_timer;

 float getRange(int pin_num){
  int sample;

  sample = analogRead(pin_num);

  return (sample);
  }

  char frameid[] = "/ir_ranger";

  void setup()
 { 
 pinMode(13, OUTPUT);
 nh.initNode();
  nh.subscribe(sub);
  nh.initNode();
  nh.advertise(pub_range);
  range_timer =  millis();                  
  }

  void loop()
  {

   if ( range_timer  + 50 <  millis() )
   {
  range_msg.range = getRange(analog_pin);
   range_msg.header.stamp = nh.now();
    pub_range.publish(&range_msg);
    range_timer =  millis();
    }

   nh.spinOnce();
    }

Circuit diagram