A switch(Touch Sensor) is added to the Feeder subsytem to control the Lego. The switch is modelled in such a way that when it is pressed for the first time, it starts calibrating all the systems in lego. After calibrating, it automatically starts to sort the ball. If the switch is pressed while sorting the ball/calibrating, it will stop its current operation(i.e Switch press now acts as a stop. When the switch is pressed once again, Lego system will start calibrating.) When the switch is pressed after ball sorting, the lego system starts to sort the ball once again.
It is wise to implement in the Material handler, because it is the master controller. But I implemented the switch control in feeder, because Brickpi of Materialhandler subsystem does not have a free port.