A level indicator for a grain bin, so you can see how full it is from the ground without climbing. Fit it through the bin wall at the level you want to watch. When the grain reaches it, the window outside turns from black to bright green, and the green glows in the dark. As the bin empties below it, the window turns back to black.
It needs no electricity. Fit several up the wall to watch key levels, for example near the bottom, near the top and at the very top.
