Now that you got an LED to blink, it's time to understand exactly how to handle inputs and outputs.
Inputs are your way of controling the Arduino; it's how you tell what you want it to do.
Some examples of inputs are buttons, potentiometers, a keypad, or even a touchscreen.
Outputs, on the other hand, allow the Arduino to tell you some information.
These can be a blinking LED, a buzzing sound, an LCD screen, ...
In the Blink example you got to see how to turn an LED on or off using digitalWrite(). Now, instead of a delay(), let's use a button.
Just like there is a digitalWrite(), there is also a digitalRead(), which returns the value of a certain digital pin (either HIGH or LOW). You use it like this:
// Assuming your button is connected to pin 2 on one end
// and to 5V on the other, the variable 'buttonState' will be
// set to HIGH or LOW according to the button
buttonState = digitalRead(2);
Now we'll make some connections, as shown below...
We can define some helpful variables, set the LED as output and the Button as input.
(setting the ledPin as OUTPUT and the buttonPin as INPUT is mandatory and must be done during the setup)
const int buttonPin = 2; // Pushbutton pin
const int ledPin = 3; // The LED pin
int buttonState = 0; // Variable for reading the button status
void setup() {
pinMode(ledPin, OUTPUT); // Initialize the LED pin as an output
pinMode(buttonPin, INPUT); // and the pushbutton pin as an input
}
Now, inside the loop, we will read the button state, and turn the LED on/off accordingly.
To do that, we will use an if(condition) {action} (which is pretty self-explainatory).
void loop() {
buttonState = digitalRead(buttonPin); // Read the state of the button
if (buttonState == HIGH) { // Check if the button is pressed
digitalWrite(ledPin, HIGH); // Turn LED on
} else {
digitalWrite(ledPin, LOW); // Turn LED off
}
}
Compile and upload. When you push the button, the LED turns on, when you release it, it turns back off.
Now imagine that you want your LED to stay on after you release the button...
Let's say we want the LED to switch state (on/off) everytime we push the button.
As the loop() funtions occurs many times/second, we can't simply switch when buttonState == HIGH, or else the LED would keep flickering when the button is pressed.
To avoid that, we need some new variables (also declared before the setup):
int ledState = HIGH; // Current state of the LED
int lastButtonState = LOW;
Inside the setup() you should also set the initial LED state.
digitalWrite(ledPin, ledState); // Set the initial LED state
Now, in every loop(), we must check the real current state of the button with digitalRead(buttonPin) and compare it with the lastButtonState. If those two are not the same, it means someone either pushed or released the button.
If the state is now HIGH, someone has just pushed the button (and we can turn the LED on or off).
You can compare a state with the previous one using the operator
!...which means "NOT":
It is also a good time to talk about debouncing. Usually, due to mechanical and physical issues, pushbuttons generate some "noise" when pressed, which might be read as multiple transitions in a short time. We won't worry too much about that for now, but keep that in mind if you are experiencing some weird results.
void loop() {
buttonState = digitalRead(buttonPin); // Read the state of the button
if (buttonState != lastButtonState) { // The button was just pressed (or released)
lastButtonState = buttonState; // Store the current button state, for the next comparison
if (buttonState == HIGH) { // It was a press, not a release
ledState = !ledState;
digitalWrite(ledPin, ledState);
}
delay(50); // A small delay helps with the random results
}
}
Now that you master digitalRead() and digitalWrite(), you can move on to the next lesson...