temperature reading not stable: please help!

Using Arduino Sensors
7

Here's something else to try. This code uses the 1.1V reference in the Arduino to calibrate what the actual value of the nominal 5V supply is. If you want to use the USB power supply, call readVcc and use the value returned instaed of the 5000 as the divisor in the temperature calculation.

int readVcc()
// Calculate current Vcc in mV from the 1.1V reference voltage
{
  long result;

  // Read 1.1V reference against AVcc
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Convert
  while (bit_is_set(ADCSRA,ADSC));
  result = ADCL;
  result |= ADCH<<8;
  result = 1126400L / result; // Back-calculate AVcc in mV
  
  return(result);
}

void setup() {
  Serial.begin(57600);
}

void loop() {
  Serial.println( readVcc(), DEC );
  delay(1000);
}