How to add control algorithm in ROS

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This is a programming question. Will you be using C++ or Python? You'll need to create a ROS Node and subscribe to the rosserial topic in it and assign the values from there. Then, on the node's callback from the topic subscription you can add an if statement that checks if the threshold is met. When the threshold is met, you can send a command down rosserial to your analog sensor make the LED glow.

See: creating a ROS node http://wiki.ros.org/ROSNodeTutorialC%... See: rosserial tutorials http://wiki.ros.org/rosserial/Tutorials See: ROS subscribers and publishers http://wiki.ros.org/ROS/Tutorials/Wri...

The function calls needed to accomplish your task can be found if you go through those tutorials. It shouldn't take long.