Button Input using Circuits GPIO

Introduction

In this exercise, we will use the circuits_gpio library to control a GPIO as an input and output, turning an LED on and off with a button press.

Try it out

Lets set a variable for the LED and button so that we dont have to choose the GPIO pin every time.

First, refer to the GPIO ports for your Nerves device.

led_pin = 20
button_pin = 17

Alright! Now that we’ve selected our pins, make sure that we connect the to our led_pin and ground through a 220Ω resistor. The long leg can be connected to our led_pin and the short pin can connect from the shorter leg to a 220Ω resistor, then to our ground.

Connect one of the legs of the button to a 10kΩ resistor to 5v, and the diagonal leg to your button_pin.

The following code will configure the led_pin as an :output and the button_pin as an input.

{:ok, led_output} = Circuits.GPIO.open(led_pin, :output)
{:ok, button_input} = Circuits.GPIO.open(button_pin, :input)

Whenever you run the following code, it will read whether your button is pressed or not, and set the led_output to match.

button_state = Circuits.GPIO.read(button_input)
Circuits.GPIO.write(led_output, button_state)

This example will listen forever for the input to change state, then toggle the output_gpio to the input_gpio state. You have to hit stop where the evaluate button used to be for this function.

# We will configure the button interrupts for both rising and falling.
Circuits.GPIO.set_interrupts(button_input, :both)

defmodule Button do
  # Watch for the button press
  def listen_forever(output_gpio) do
    receive do
      {:circuits_gpio, _p, _timestamp, 1} ->
        Circuits.GPIO.write(output_gpio, 1)

      {:circuits_gpio, _p, _timestamp, 0} ->
        Circuits.GPIO.write(output_gpio, 0)
    end

    listen_forever(output_gpio)
  end
end

Now we can call the Button.listen_forever function with our led_output as the argument.

Button.listen_forever(led_output)

For a fun exercise, see if you can update the Button.listen_forever function to only change the led_output state on :rising only.