Lloyd Rochester's Geek Blog

Blinking an LED with libgpiod

In the previous post we introduced libgpiod. In this post we’ll use the command line tools to blink an LED. Finally, we’ll write some C code to blink the LED using the libgpiod library.

Table of Contents

LED Driver Circuit

Skip this section if you already have a good circuit to drive an LED. A simple NPN Transistor is a cheap and robust LED drive that only requires two resistors.

LED Drive Circuit

Let’s say you have an LED like this. This LED can take a maximum of \( 30mA \), so let’s reduce to \( 20mA \). The LED has a Forward Voltage of \( V_f=1.8V \) typical. The current going through the LED is:

\( 20mA = \frac{5V - 1.8V}{ R_2 } \)

Solving for \( R_2 \) we have:

\( R_2 = \frac{3.2V}{20mA} = 160\Omega \).

For \( R_1 \) let’s just drive \( 1mA \) out the GPIO Pin 21 so we have:

\( 1mA = \frac{3.3V - 0.7V}{R_1} \).

Solving for \( R_1 \) we have:

\( R_1 = \frac{2.6V}{16mA} = 2600 \Omega \).

For both values \( R_1 \) and \( R_2 \) rounding down gives less current and going up risks damaging the LED and Raspberry Pi. Thus, we can use \( 2.7k\Omega \) for a standard value. The value of \( 160 \Omega \) is already standard.

You can easily plug the values for a different LED into the equations above to get the right resistor values.

Blinking the LED with gpioset

We can turn the LED on using the gpioset command:

$ gpioset gpiochip0 21=1

and turn it off again with:

$ gpioset gpiochip0 21=0

or we can turn it on for a duration: 

$ gpioset --mode=time --mode=time --sec=3 gpiochip0 21=1

C Code to Blink the LED

// file blink.c
#include <gpiod.h>
#include <error.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

struct gpiod_chip *chip;
struct gpiod_line_request_config config;
struct gpiod_line_bulk lines;

int
main(int argc, char *argv[])
{
  unsigned int offsets[1];

  int values[1];
  int err;

  chip = gpiod_chip_open("/dev/gpiochip0");
  if(!chip)
  {
    perror("gpiod_chip_open");
    goto cleanup;
  }

  // set pin 21 to 1 (logic high)
  offsets[0] = 21;
  values[0] = 0;

  err = gpiod_chip_get_lines(chip, offsets, 1, &lines);
  if(err)
  {
    perror("gpiod_chip_get_lines");
    goto cleanup;
  }

  memset(&config, 0, sizeof(config));
  config.consumer = "blink";
  config.request_type = GPIOD_LINE_REQUEST_DIRECTION_OUTPUT;
  config.flags = 0;

  // get the bulk lines setting default value to 0
  err = gpiod_line_request_bulk(&lines, &config, values);
  if(err)
  {
    perror("gpiod_line_request_bulk");
    goto cleanup;
  }

  // output value 1 to turn on the led
  values[0] = 1;
  err = gpiod_line_set_value_bulk(&lines, values);
  if(err)
  {
    perror("gpiod_line_set_value_bulk");
    goto cleanup;
  }

  sleep(1);

  // output value 0 to turn off the led
  values[0] = 0;
  err = gpiod_line_set_value_bulk(&lines, values);
  if(err)
  {
    perror("gpiod_line_set_value_bulk");
    goto cleanup;
  }

cleanup:
  gpiod_line_release_bulk(&lines);
  gpiod_chip_close(chip);

  return EXIT_SUCCESS;
}

Building and Running

It’s easy to build and run:

$ gcc -o blink blink.o -lgpiod
$ ./blink

You should see an LED blink for 3 seconds.

#raspberry-pi