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14 Νοεμβρίου 2014


Filed under: ARDUINO — admin @ 13:52

const int sensorPin = A0;
const float baseLineTemp = 20.0; //float values can store decimals.

void setup() {
  Serial.begin(9600); //open a serial port

  for(int pinNumber = 2; pinNumber<6; pinNumber++){
    pinMode(pinNumber, OUTPUT);
    digitalWrite(pinNumber, LOW);

void loop() {
  int sensorVal = analogRead(sensorPin);

  Serial.print(“Sensor Value: “);

  //convert the ADC reading to voltage
  float voltage = (sensorVal/1024.0) * 5.0;

  Serial.print(“, Volts: “);
  Serial.print(“, degrees C:”);
  //convert the voltage to temperature in degrees
  float temperature = (voltage – .5) * 100;

  if(temperature <= baseLineTemp){
    digitalWrite(2, HIGH);
    digitalWrite(3, LOW);
    digitalWrite(4, LOW);
    digitalWrite(5, LOW);

  }else if(temperature >= baseLineTemp+2 && temperature <baseLineTemp+4){
    digitalWrite(2, LOW);
    digitalWrite(3, HIGH);
    digitalWrite(4, LOW);
    digitalWrite(5, LOW);
  }else if(temperature >= baseLineTemp+4 && temperature < baseLineTemp+8){
    digitalWrite(2, LOW);
    digitalWrite(3, HIGH);
    digitalWrite(4, HIGH);
    digitalWrite(5, LOW);
  }else if(temperature >= baseLineTemp+8){
    digitalWrite(2, LOW);
    digitalWrite(3, HIGH);
    digitalWrite(4, HIGH);
    digitalWrite(5, HIGH);

I’m just trying out Arduino Uno for the first time with 2 blinking LEDs on a breadboard. All the tutorials on the Internet seem to use a resistor. I do know the function of resistors, but does it really matter here? These LEDs are working just fine without a resistor.
shareimprove this question
(if you would ever return here): don’t hit and run. You asked a question, got an answer, and you were gone. You haven’t upvoted the answer (you didn’t have enough reputation) and you haven’t accepted it. Since you don’t seem to come back I suppose your question has been answered. Give credit where credit’s due. –  Federico Russo Jun 3 ’12 at 9:38
The good part is that person cared to ask. So future generations are now able to find the question and answer –  user924 Jun 4 ’12 at 2:48
Ok. Sorry about that, I’m new here. –  40Plot Jun 14 ’12 at 13:34

1 Answer

up vote 32 down vote accepted
Naughty! :-). If they say to use a resistor there’s a good reason for that! Switch it off, NOW!
The resistor is there to limit the LED’s current. If you omit it the current limiting has to come from the Arduino’s output, and it will not like it. How do you find out what the resistor needs to be? You do know Ohm’s Law? If you don’t, write it down in big letters:
Voltage equals current times resistance. Or you could say
It’s the same thing. The voltage you know: Arduino runs at 5V. But not all that will go over the resistor. The LED also has a voltage drop, typically around 2V for a red LED. So there remains 3V for the resistor. A typical indicator LED will have a nominal current of 20mA, then
The Arduino Uno uses the ATmega328 microcontroller. The datasheet says that the current for any I/O pin shouldn’t exceed 40mA, what’s commonly known as Absolute Maximum Ratings. Since you don’t have anything to limit the current there’s only the (low!) resistance of the output transistor. The current may so well be higher than 40mA, and your microcontroller will suffer damage.
The following graph from the ATmega’s datasheet shows what will happen if you drive the LED without current limiting resistor:
enter image description here
Without load the output voltage is 5V as expected. But the higher the current drawn the lower that output voltage will be, it will drop about 100mV for every extra 4mA load. That’s an internal resistance of 25Ω. Then
The graph doesn’t go that far, the resistance will rise with temperature, but the current will remain very high. Remember that the datasheet gave 40mA as Absolute Maximum Rating. You have three times that. This will definitely damage the I/O port if you do this for a long time. And probably the LED as well. A 20mA indicator LED will often have 30mA as Absolute Maximum Rating.

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