I read it as a request to stop and start the feed conveyor/screw so the bin fills up again when it reaches empty. There must be an outflow to the mill allowing it to empty as well.
If I'm right, there are several solutions to the problem but which is best will depend on how the level sensor can be attached to the bin. The ideal method would be to weigh the contents and use a simple weight operated switch but the mechanical aspects may not allow that. My second solution would be to use an optical sensor at the top and one at the bottom of the bin, if the light beam is interrupted by the grain you would know it was at least at that level. A simple flip-flop triggered by the sensors would say "I'm empty" or "I'm full" and operate the clutch accordingly.
It is also possible to do this with ultrasonics from a single sensor above the bin, detecting the level rather like a reversing sensor on a car detects the presence of an obstruction behind it but if the grain absorbs the sound waves or produces 'noisy' reflections it may not work reliably. There is also more complicated electronics involved although only a little more expensive than the other methods.
just ideas...
Brian.