You now have both a sensor and an actuator that speaks MQTT! Great. But they still don't communicate with anyone. To test your communication interfaces, there are many client tools available for download on the Internet. Some will allow you to publish and subscribe to data through interactive GUIs. The following screenshot shows how
mqtt-spy (https://github.com/eclipse/paho.mqtt-spy/wiki/Downloads) can be used to interact with our actuator and monitor general MQTT activity:
Using mqtt-spy to interact with our Actuator.