Arduino connects the switch

button or switch to connect the two open terminals in the circuit. /b10> This example turns on the LED on pin 2 when the button switch connected to pin 8 is pressed.

Drop-down resistor

Use a pull-down resistor in an electronic logic circuit to ensure that Arduino's input signal reaches the expected logic level when the external device is disconnected or in a high impedance state. A lthough nothing is connected to the input pin, this does not mean that it is a logic 0. /b11> The pull-down resistor is connected between the ground and the corresponding pins on the device.

The following illustration shows an example of a drop-down resistor in a digital circuit. /b10> A button switch is connected between the supply voltage and the microcontroller pin. /b11> In such a circuit, the microcontroller input is at a logic high value when on and off, but when the switch is on, the drop-down resistor pulls the input voltage down to ground (logical zero value) to prevent the input from being in an undefined state.

The pull-down resistance must be greater than the impedance of the logic circuit, otherwise the voltage may drop too much, and the input voltage at the pin will remain at a constant logic low, regardless of the position of the switch.

The required component

You will need the following components:

• 1 × Arduino UNO board
• 1 × 330 ohm resistor
• 1 × 4.7K ohm resistor (pull-down)
• 1 × LED

Program

Connect according to the circuit diagram, as shown in the following image.

Sketch

Turn on the Arduino IDE software on your computer. U se arduino to encode and control your circuitry. /b11> Open a new sketch file by clicking New.

Arduino code

// constants won't change. They're used here to
// set pin numbers:
const int buttonPin = 8; // the number of the pushbutton pin
const int ledPin = 2; // the number of the LED pin
// variables will change:
int buttonState = 0; // variable for reading the pushbutton status

void setup() {
   // initialize the LED pin as an output:
   pinMode(ledPin, OUTPUT);
   // initialize the pushbutton pin as an input:
   pinMode(buttonPin, INPUT);
}

void loop() {
   // read the state of the pushbutton value:
   buttonState = digitalRead(buttonPin);
   // check if the pushbutton is pressed.
   // if it is, the buttonState is HIGH:
   if (buttonState == HIGH) {
      // turn LED on:
      digitalWrite(ledPin, HIGH);
   } else {
      // turn LED off:
      digitalWrite(ledPin, LOW);
   }
}

Code description

When the switch is on (the button is not pressed), there is no connection between the two terminals of the button, so the pin is grounded (via a drop-down resistor) and we read low. /b10>When the switch closes (the button is pressed), it establishes a connection between its two terminals, connecting the pins to 5 volts, so that we can read high.

Results

The LED lights up when the button is pressed and the LED lights off when the button is released.