Originally Posted by
RomanLini
Ultrasonic is very poor on something fluffy or energy absorbent, I think it would not work well.
I remember a similar thread somewere and someone suggested a simple light bulb type source on one side, and a simple light sensor on the other side. That will tolerate most swirling airborne dust pretty well, but finally when the sensor is buried deep enough in dust it will be "fully dark" and trigger the alarm.
Sounds like the easiest option.
You could mount the sensor on the underside of the lid and an Led (as doesn't produce heat to speak of) on the side wall at the level you want. Dust gets above that level and completely covers the led and the sensor trips.
7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)