I have one I got from Penn state. Coincidentally it just quit working a couple days ago. Besides that it works fine. It has a plastic paddle that turns continuously until it meets resistance from the rising level of sawdust. when the paddle meets resistance or stops, it sounds a buzzer or it can be set up to trigger a light. I just left mine with the internal buzzer. Mine is on Penn state's 35 gallon drum. It does go off before the drum is full. I have been planning on experimenting with the paddle to shorten it so I doesn't buzz as early. I just haven't taken the time to experiment.
Also I haven't taken the time yet to see why mine quit working. It buzzes still with resistance on the paddle, but it quit turning. I'm not sure if the motor that turns it is shot or if something is loose. I've had it about 5 years and it is turning all the time when my shop lights are on. They are 79.95 and you can get a 110 or 220 model depending on your needs. Mine is 110 and is plugged into a socket on the overhead lights.
I am happy with how it works and will probably replace it with the same thing unless someone chimes in on here with a better idea. I'm not familiar with anything else that is available.
One suggestion. If you get one and mount it, keep it away form the outside of the drum. I use bags inside my drum which are retained to the wall of the drum with a sheet of thick plastic rolled into a circle (also sold at penn state). I mounted my sensor a little too close and if the retaining piece of plastic isn't tight to the wall, it can hit the sensor paddle making it buzz. Usually this happens when you install a new bag. Once you adjust it correctly it is fine. But If I had put the sensor a little closer to the center it would be more fool proof.
Hope that helps.