How to Control an LED with ROS and ESP32

Introduction

In this tutorial, we'll learn how to control an LED using the Robot Operating System (ROS) and an ESP32 development board. ROS is a popular framework for building robotics applications, and it provides a convenient way to communicate between different nodes in a system. We'll use ROS to send commands to the ESP32, which will turn an LED on or off.

Prerequisites

Before we get started, you'll need the following:

• An ESP32 development board
• A breadboard
• An LED
• A 220 ohm resistor
• Jumper wires
• A computer running Ubuntu 20.04 with ROS installed

Setting up the Hardware

First, let's connect the LED to the ESP32. Insert the longer leg of the LED into pin 2 on the ESP32, and the shorter leg into the breadboard. Connect one end of the resistor to the same row as the shorter leg of the LED, and the other end to the ground rail on the breadboard.

Creating a ROS Node

Next, let's create a ROS node that will control the LED. Here's the code we'll use:

arduino
#include <ros.h>
#include <std_msgs/Empty.h>

ros::NodeHandle nh;

const int LED_PIN = 2;

void messageCb(const std_msgs::Empty& toggle_msg) {
  digitalWrite(LED_PIN, HIGH - digitalRead(LED_PIN)); // blink the led
}

ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb);

void setup() {
  pinMode(LED_PIN, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

void loop() {
  nh.spinOnce();
  delay(1);
}

This code listens for messages on the /toggle_led topic, and toggles the LED on or off when a message is received. We'll publish messages to this topic later to control the LED.

Save this code to a file called toggle_led.cpp, and compile it using the following command:

css
$ arduino-cli compile --fqbn esp32:esp32:esp32 toggle_led.cpp

This will generate a binary file called toggle_led.esp32.esp32.esp32.bin.

Uploading the Binary to the ESP32

Now, let's upload the binary to the ESP32. Connect the ESP32 to your computer via USB, and run the following command:

css
$ arduino-cli upload -p /dev/ttyUSB0 --fqbn esp32:esp32:esp32 toggle_led.cpp

This will upload the binary to the ESP32, and start the toggle_led node.

Controlling the LED

Finally, let's send messages to the /toggle_led topic to turn the LED on or off. Open a new terminal window, and run the following command to turn the LED on:

rust
$ rostopic pub /toggle_led std_msgs/String "data: 'on'"

The LED should turn on. To turn it off, run the following command:

rust
$ rostopic pub /toggle_led std_msgs/String "data: 'off'"

And that's it! You should now be able to control the LED using ROS and the ESP32.

Conclusion

In this tutorial, we learned how to control an LED using ROS and an ESP32 development board. We created a ROS node that listens for messages on the /toggle_led topic, and toggles the LED on or off when a message is received. We then uploaded the binary to the ESP32, and sent messages

arduino
#include <ros.h>
#include <std_msgs/Empty.h>

ros::NodeHandle nh;

const int LED_PIN = 2;

void messageCb(const std_msgs::Empty& toggle_msg) {
  digitalWrite(LED_PIN, HIGH - digitalRead(LED_PIN)); // blink the led
}

ros::Subscriber<std_msgs::Empty> sub("toggle_led", &messageCb);

void setup() {
  pinMode(LED_PIN, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

void loop() {
  nh.spinOnce();
  delay(1);
}

And here is the modified listener.py code to turn the LED on and off using the rostopic pub command:

python
#!/usr/bin/env python

import rospy
from std_msgs.msg import String

LED_PIN = 2

def callback(data):
    if data.data == "on":
        rospy.loginfo("Turning LED on")
        GPIO.output(LED_PIN, GPIO.HIGH)
    elif data.data == "off":
        rospy.loginfo("Turning LED off")
        GPIO.output(LED_PIN, GPIO.LOW)

def listener():
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber("toggle_led", String, callback)
    rospy.spin()

if __name__ == '__main__':
    try:
        GPIO.setmode(GPIO.BOARD)
        GPIO.setup(LED_PIN, GPIO.OUT)
        listener()
    except rospy.ROSInterruptException:
        pass