MQTT and Raspberry Pi GPIO

gpio-pins-diagramThe Raspberry Pi has 40 General purpose Input/output Pins (GPIO) that can be used for controlling external hardware.

There are  a number of add-on boards called hats which plug into these pins and provide various sensors. The most common one being the sense hat.



Although you will need external hardware like leds etc for various projects there is a lot you can learn without using any.

In this tutorial I will cover the basics of using the GPIO pins and show you how to use MQTT to set the pin state and report the pin state using Python..

PIN or Channel Numbers

This aspect I found very confusing at first as there are two systems that are used, and you need to choose which one to use in your scripts.

The systems are:

  • Board- uses numbers 1-40 and follows the pin Layout.
  • BCM – uses designations line GPIO4

GPIO4 corresponds to Board Pin number 7 as shown in the diagram below:gpio-pins-diagram

Note: diagram copied from this page which contains complete pin layout

Reference reading:

Simple guide to the Raspberry pi GPIO pins

Controlling The Pins Using Python

There are two modules that can be used for controlling i.e reading and writing to the pins. They are:

Both modules are normally installed but the install procedures can be found on the reference pages above.

Using

To use you will need to import them. The code is

import RPi.GPIO as GPIO

or

import gpiozero

Using The RPI.GPIO Module

To use a pin you need to first set the numbering scheme and then designate the Pin as either an input or an output pin.

Using the BCM format we use:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM) #set numbering scheme
GPIO.setup(4,GPIO.OUT) #set GPIO4 as output.
GPIO.setoutput(4,1) #set value of GPIO4 to True or 1

to set a pin as an input pin use:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM) #set numbering scheme
GPIO.setup(5,GPIO.IN) #set GPIO5 as input.

You can read the value of a pin using:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM) #set numbering scheme
GPIO.setup(4,GPIO.OUT) #set GPIO4 as output.
GPIO.setoutput(4,1) #set value of GPIO4 to True or 1
GPIO.setup(5,GPIO.IN) #set GPIO5 as input.
pin5=GPIO.input(5) #Read value GPIO5.
pin4=GPIO.input(4) #Read value GPIO4.

Note: GPIO.BCM,GPIO.OUT etc are constants and pins are often called channels when using the BCM scheme.

Test Script

The following script demonstrates the above:

#! /usr/bin/python3.5
import sys
import RPi.GPIO as GPIO
import paho.mqtt.client as mqtt
cpio_channels=[4,5,6,12,13,16,17,18,19,20,21,22,23,24,25,26,27]# base channels
print("using ",sys.version) #what version of python
mode=GPIO.getmode()
print("mode is ",mode)
GPIO.setmode(GPIO.BCM)
mode=GPIO.getmode()
print("mode is ",mode)
GPIO.setup(4,GPIO.OUT)
GPIO.output(4,1)
print("value is ",GPIO.input(4))
GPIO.setup(5,GPIO.IN)
print("input value is ",GPIO.input(5))
GPIO.cleanup()
print("board ",GPIO.BOARD)
print("bcm ",GPIO.BCM)

Connecting to MQTT

To publish the value of a pin we first need to decide on a topic naming scheme.

To set the value of the pin we also need a control topic.

Below I use:

  • pi/GPIO/channel_number
  • pi/GPIO/control/channel_number

To publish on pin/channel 4 (GPIO4) we use

#First setup MQTT See later

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM) #set numbering scheme
GPIO.setup(4,GPIO.OUT) #set GPIO4 as output.
GPIO.setoutput(4,1) #set value of GPIO4 to True or 1
mqtt.publish("pi/GPIO/4",GPIO.input(4))

Controlling Pins using MQTT

The script below is available as a down load will let you control an output pin/channel using MQTT.

To test I used the mosquitto_pub tool with a payload of 1 or 0

e.g

mosquitto_pub -h 192,168.1.158 -t pi/GPIO/control/4 -m 1

will set the pin/channel to 1.
The script will publish the channel/pin value on
pi/GPIO/4.

So you can subscribe to this topic and see it change as you send the commands.

FeedBack

I created this tutorial as a response to a question I received. I never intended to use the GPIO pins myself. Therefore I would be grateful for any feedback you may have.

Related tutorials and resources:

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