Random Numbers

Random numbers are useful in many Arduino projects — from rolling dice in games, to generating random LED patterns, to adding unpredictability in robotics. Arduino provides two functions for working with random numbers: random() to generate them and randomSeed() to make them less predictable.

random()

The random() function generates a pseudo-random number. It has two forms: random(max) returns a random number between 0 and max - 1, while random(min, max) returns a random number between min and max - 1.

void setup() {
  Serial.begin(9600);
  // Generate a random number between 0 and 9
  int val1 = random(10);
  Serial.print("0-9: ");
  Serial.println(val1);

  // Generate a random number between 5 and 14
  int val2 = random(5, 15);
  Serial.print("5-14: ");
  Serial.println(val2);
}

void loop() {
  // nothing to do here
}

In this example, random(10) produces a value from 0 to 9, and random(5, 15) produces a value from 5 to 14. Each time you reset the board, however, you will see the same sequence of numbers. This happens because the pseudo-random generator starts from the same default seed every time.

You can use random() inside loop() to create dynamic behavior:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int delayTime = random(200, 2000);
  digitalWrite(LED_BUILTIN, HIGH);
  delay(delayTime);
  digitalWrite(LED_BUILTIN, LOW);
  delay(delayTime);
  Serial.print("Random delay: ");
  Serial.println(delayTime);
}

This sketch blinks the built-in LED with a random on/off delay between 200 ms and 1999 ms, creating an unpredictable blinking pattern.

randomSeed()

The randomSeed() function initializes the pseudo-random number generator with a seed value. Without calling randomSeed(), the generator produces the same sequence of numbers every time the board starts. By seeding it with a varying value — typically from an unconnected analog pin — you get different sequences on each run.

void setup() {
  Serial.begin(9600);
  // Seed the random generator with noise from an unconnected pin
  randomSeed(analogRead(A0));
}

void loop() {
  int val = random(1, 7);
  Serial.println(val);
  delay(1000);
}

In this example, randomSeed(analogRead(A0)) reads the floating voltage noise from analog pin A0 (which should be left unconnected) and uses that value as the seed. This produces a different sequence of numbers each time the board is powered on. The sketch then simulates rolling a six-sided die every second, printing values from 1 to 6.

You should call randomSeed() only once in setup(). Calling it repeatedly or before every random() call can actually make the output less random, since analogRead() may return similar values in quick succession.