Blink using Circuits GPIO

Introduction

In this exercise, we will use the circuits_gpio library to control a GPIO as an output, and blink an LED.

Try it out

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

First, refer to the GPIO ports for your Nerves device. In this example, we will be using pin 20 on the raspberry pi platform.

led_pin = 20

Alright! Now that we’ve selected a pin, 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.

Now we will create a connection to that GPIO pin and set it as an :output.

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

After our led is wired to our GPIO pin, lets toggle it on:

Circuits.GPIO.write(led_output, 1)

Then off:

Circuits.GPIO.write(led_output, 0)

To take this one step further, we can write a short recursive function to blink the light on and off with a certain delay.

defmodule Blink do
  # Blink forever
  def forever(output_gpio, delay \\ 1000) do
    Circuits.GPIO.write(output_gpio, 1)
    Process.sleep(delay)

    Circuits.GPIO.write(output_gpio, 0)
    Process.sleep(delay)

    forever(output_gpio, delay)
  end
  # Default just giving a pin will match here and run at 1 second.
  def pin(output_gpio) do
    pin(output_gpio, 500, 3)
  end
  # Or provide a delay to blink faster or slower
  def pin(_output_gpio, _delay, times) when times <= 0  do
    # Do nothing after the count completes
  end

  def pin(output_gpio, delay, times) do
    Circuits.GPIO.write(output_gpio, 1)
    Process.sleep(delay)

    Circuits.GPIO.write(output_gpio, 0)
    Process.sleep(delay)
    pin(output_gpio, delay, times - 1)
  end
end

We can blink it using the default 1 second delay.

Blink.pin(led_output)

Or with a set delay. This will blink 10 times every 100ms.

Blink.pin(led_output, 100, 10)

Lastly, we can blink forever. You have to hit stop where the evaluate button used to be for this function.

Blink.forever(led_output)

For a fun exercise, see if you can change the Blink.forever function to blink at 100ms instead of 1000ms.