We have a cat food dispenser that stores the food vertically in a silo type tower with an opening at the bottom where the food spills out into a small dish. We keep this tower in a closet on the floor under a shelf. The light in the closet does not illuminate the tower enough to see the food level. This device lights up when the food level runs low.
The device is mounted at the top of the tower. Inside the tower, a golf ball attached to the device with a piece of string monitors the food level.
When the ball lowers to what the kitties refer to as dangerously low food level (anything but full), the string goes taught causing a light to indicate the food level is unacceptably low. When a human servant (i.e. me or my wife) see the light we know we must obey the kitty rulers and refill the food tower immediately.