I've been playing around with the Arduino world of electronics bits and pieces and decided an alarm to alert me to a full dust collector would be a worthwhile little project. The alarm uses an ultrasonic range sensor to detect the level of the sawdust in the bag. A piezo buzzer beeps when the level is within about 12" of the sensor, sounds continuously when the level is within 6". Total cost was about $5.
Here's the completed unit mounted on the dust collector in a bracket a friend 3D printed. (He works at a 3d printer company and free printing is an official fringe benefit).
Here's what the guts look like spread out -
And here they are jammed in the box:
and the sensor inside the dust collector: