analog reference

Using Arduino Microcontrollers
6

I am doing this as my college project.
I am using sim900 module along with PIR sensor.

void loop()
{
    val1 = digitalRead(inputPin1); // read input value from PIR
    if (val1 == HIGH) { // check if the input is HIGH
      mySerial.print("ATD +919*********;");//dial the number
      delay(100);
      mySerial.println();
  } 
  else {
  }
}

now this code calls me even when nothing is detected by PIR.
And for power,I am using 3.7V battery.