Let's now create a small project for the Raspberry Pi, that uses ROS to turn on or off a led connected to it's GPIO's.

For this, we'll need a publisher node that acts like the brain of our system. The calculation and decisions are done on this node. The subscriber node will intercept the message and depending it's value, it will turn the led on or off.

Create a python program and name it controller.py.

We'll create a node called controller and send the led value to the led_value topic. This value will be incremented every second.

import rospy
from std_msgs.msg import Int

count = 0

def talker():
    pub = rospy.Publisher('led_value', Int, queue_size=10)
    rospy.init_node('controller', anonymous=True)
    rate = rospy.Rate(1)
    while not rospy.is_shutdown():
        count += 1
        pub.publish(count)
        rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

Create a python program and name it led1.py. We'll create a node called led1 and intercept the led_value topic. If it's value is even, it will turn the led on. If it's odd, the led is turned off.

import rospy
import RPi.GPIO as GPIO
from std_msgs.msg import Int

count = 0

def callback(data):
    if data.data % 2:
      GPIO.output(18,GPIO.HIGH)
    else:
      GPIO.output(18,GPIO.LOW)

def listener():
    rospy.init_node('led1', anonymous=True)
    rospy.Subscriber('led_value', Int, callback)
    rospy.spin()

if __name__ == '__main__':
    try:
        GPIO.setmode(GPIO.BCM)
        GPIO.setup(18,GPIO.OUT)
        listener()
    except rospy.ROSInterruptException:
        pass