For this recipe, ensure that you have the Arduino IDE running on a computer.

The following code turns a pin HIGH and LOW repeatedly while reading the external voltage applied to another:

void setup() {
  // Set pin 2 as a digital Output
  pinMode(2, OUTPUT);
  // Set pin 3 as a digital Input
  pinMode(3, INPUT);
}

void loop(){
    // Set pin 2 HIGH
  digitalWrite(2, HIGH);
  // Wait 100 milliseconds
  delay(100);
  // Set pin 2 LOW
  digitalWrite(2, LOW);
  // Wait 100 milliseconds
  delay(100);
    // Read the value of pin 3 and store it in a variable
  int pinValue = digitalRead(3);
}

The code sets two pins in output and input mode and then writes and reads from them. Here is the code breakdown:

In setup(), we use the pinMode() function to set pin number 2 as an output. When we set a pin as an output, we can set that pin as either HIGH (5 V) or LOW (0 V). Also, we set pin number 3 as an input. A pin configured as input can read external voltages applied to it. It can read HIGH if the voltage is around 5 V and LOW if the voltage is close or equal to 0 V:

void setup() {
  // Set pin 2 as a digital Output
  pinMode(2, OUTPUT);
  // Set pin 3 as a digital Input
  pinMode(3, INPUT);
}

In the loop() function, we use the digitalWrite() function to set pin number 2 to HIGH. Then, we wait for 100 milliseconds using the delay() function. This function stops the execution of the code for the specified time, in milliseconds. Thereafter, we set the pin to LOW and wait another 100 milliseconds. In the end, we read the value of pin 3 in a variable:

void loop(){
  
  // Set pin 2 HIGH
  digitalWrite(2, HIGH);
  // Wait 100 milliseconds
  delay(100);
  // Set pin 2 LOW
  digitalWrite(2, LOW);
  // Wait 100 milliseconds
  delay(100);
  
  // Read the value of pin 3 and store it in a variable
  int pinValue = digitalRead(3);
}