Read a button with Arduino

In the tutorial Digital Inputs in Arduino we saw how to read a sensor that provides a digital signal with two voltage levels, LOW and HIGH. We left pending how to use digital inputs to read the state of a switch or push button with Arduino.

To do this, we will need to correctly perform a specific setup and the help of two new friends: Pull Down and Pull Up resistors. Although both cases are very similar, the setup and the type of resistor to use will depend on whether we want to read a LOW or HIGH value when the button or switch is pressed.

To understand how both setups work, we will present the logical reasoning that leads to them. For this, we will use two incorrect setups that will help us understand the role of each element in the final, correct setup.

So, let’s start with the first attempt to read the button state.

First Attempt, Direct Connection

Our first idea to read a button could be to directly connect a digital PIN of Arduino to a reference voltage, be it 0V or 5V.

When the button is closed, the voltage at the PIN would be the reference value (0V or 5V depending on the setup) and we could perform the reading as with any digital input.

What’s the problem? Well, this will work when the switch is closed. But, what happens when the switch is open? In this case, we are leaving the PIN completely disconnected from any voltage (something we will call a high impedance state).

What value does an input register if we take a measurement in a high impedance state? It depends on several factors, such as the internal construction of the input or the last state it was connected to.

How can we resolve this indeterminate state? Well, this leads us directly to our second attempt. 👇

Improving Our Solution, Double Connection

The next thing we might think is to connect the PIN to two voltage references, alternated based on the switch state:

To measure a LOW value when the switch is pressed

• We can connect the PIN fixed to 5V, and to 0V through the switch.
• With the switch open we would read HIGH.
• When the switch is closed, 0V would be forced on the PIN, so we would read LOW.

To measure a HIGH value when the switch is pressed

• We can connect the PIN fixed to 0V, and to 5V through the switch.
• With the switch open we would read LOW.
• When the switch is closed, 5V would be forced on the PIN, so we would read HIGH.

What’s the problem with this setup? Well, when pressing the switch we are directly connecting the 0V and 5V values, which means we are causing a short circuit.

This would cause a high current flow and rapid heating of components and conductors (death, destruction, getting fired…).

How to avoid this short circuit? Well, we are close. We’ll see this next in the definitive setup.

Correct Setup, Pull-Down or Pull-Up Resistors

As we had anticipated, to correctly solve the setup we will need the presence of two new friends, Pull Down and Pull Up resistors. These two resistors are a basic mechanism, very common in the world of electronics and automation.

Pull-Down and Pull-Up resistors are connected between the digital PIN and one of the reference voltages (0V or 5V) and “force” or “pull” (hence their name) the voltage value to LOW or HIGH, respectively.

This is how the final setup would look in a schematic view (the connection can be made using any of the digital PINs).

And this is the wiring on a breadboard.

Finally, reading the state of the PIN is done normally, just as we saw in the tutorial Digital Inputs in Arduino.

const int inputPin = 2;

int value = 0;
 
void setup() {
  Serial.begin(9600);
  pinMode(inputPin, INPUT);
}
 
void loop(){
  value = digitalRead(inputPin);  //digital reading of pin
 
  //send message to serial port based on the value read
  if (value == `HIGH`) {
      Serial.println("On");
  }
  else {
      Serial.println("Off");
  }
  delay(1000);
}

Try it online

Download the Code

All the code from this post is available for download on Github.