2.2 Controlling LEDs with ROS 2

Next, we'll integrate two LEDs with the Arduino. These will be controlled through ROS commands.

2.2.1 Hardware Setup

You'll need:

• Red LED

• Blue LED

• 2 x 220Ω resistors

• Jumper wires

• Breadboard

Wiring:

• Connect the Red LED to digital pin 7 (with the resistor in series).

• Connect the Blue LED to digital pin 8 (with the resistor in series).

• Connect the grounds to the Arduino GND pin.

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2.2.2 Writing ROS Node for Serial Communication

The first step is writing a simple ROS node in Python that sends a command over serial to the Arduino.

Create a ROS package if you haven’t already:

source /opt/ros/foxy/setup.bash
cd ~/ros_ws/src
ros2 pkg create --build-type ament_python --node-name ser_leds ros_arduino_leds

You will now have a new folder within your workspace’s src directory called ros_arduino_basic.

cd ~/ros_ws/src/ros_arduino_leds/ros_arduino_leds
nano ser_leds.py

Here’s a simple ROS node that communicates with the Arduino:

ser_leds.py:

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
import serial

class ArduinoLeds(Node):
    def __init__(self):
        super().__init__('arduino_leds')
        self.ser = serial.Serial('/dev/ttyUSB0', 9600, timeout=1)
        self.ser.flush()

        self.create_timer(2, self.send_command)

    def send_command(self):
        command = input("Enter 1 (Red ON), 2 (Blue ON), 0 (All OFF): ")
        self.ser.write(command.encode())
        self.get_logger().info(f"Sent command: {command}")

def main(args=None):
    rclpy.init(args=args)
    node = ArduinoLeds()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Make sure to modify /dev/ttyUSB0 to match the correct serial port for your Arduino.

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2.2.3 Uploading Arduino Code

Now, upload a simple Arduino sketch to receive data from ROS.

Arduino Sketch:

#define RED_LED 7
#define BLUE_LED 8

void setup() {
  Serial.begin(9600);
  pinMode(RED_LED, OUTPUT);
  pinMode(BLUE_LED, OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    char received = Serial.read();
    if (received == '1') {
      digitalWrite(RED_LED, HIGH);
      Serial.println("Red LED ON");
    } else if (received == '2') {
      digitalWrite(BLUE_LED, HIGH);
      Serial.println("Blue LED ON");
    } else if (received == '0') {
      digitalWrite(RED_LED, LOW);
      digitalWrite(BLUE_LED, LOW);
      Serial.println("LEDs OFF");
    }
  }
}

Upload this code to your Arduino using the Arduino IDE.

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2.2.4 Running the Code

1. Open a terminal on your PC and source ROS 2:

   source /opt/ros/foxy/setup.bash
   cd ~/ros_ws
   colcon build
   source install/setup.bash

2. Run the ROS node:

   ros2 run ros_arduino_leds ser_leds

Now you can see commands being sent from ROS to Arduino. Check the Arduino Serial Monitor to see the received data.

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