Although I think the ASD (seems like a wierd name) is a neat device, it isn't quite right for me.
Where it is positioned it is going to overfill my barrel. I don't want to install it on the barrel because I haul it out to empty it. Any chance you can design a paddle style like Penn State now has that mounts into the lid?
Travis, the DustBowlBoys have a couple of ideas...
If you could extend a sleeve an inch below the lid, as the barrel approaches full, it would not deposit chips in the "freeboard" between the bottom of the sleeve and top of lid, and would collect in the sleeve. When the sensor fired, you would have some surplus room in the barrel to catch the sleeve buildup. I think there's a standard HVAC thingie (collar or takeoff) that you could just caulk to the bottom of the lid that would act as a sleeve.
There's also been talk of using a periscope system, with two mirrors mounted on 45° brackets to shoot the light down, project the beam between mirrors a couple of inches down, and then reflect it back up to the sensor. All of this would be lid-mounted and not affect the barrel. I don't think this concept has been tested.
Also, there's a complete write-up and schematic for a $20 (parts only, doesn't include shipping) paddle sensor on Bill's website. When I couldn't find a fast acting photosensor, I contemplated building one of these. In retrospect, I am much happier with my ADS unit. If you're willing to spring the $$$ for the parts, I'm willing to help you assemble it.
And finally, shouldn't it be ADS? That would be an AlanDustSensor. What's an ASD?