This is an LED Filament Indicator Extruder Cover for MK3S. It houses a small 3mm LED that uses the ground and signal wires of the filament sensor. The circuit is extremely simple: simply wire a 3V LED to a 100k Ohm resistor and connect the two wires to the back of the filament sensor plug. As you can see from the picture with the power supply and the amp meter, a 3.6V white LED and a 100k Ohm resistor will draw just 25.9 micro-ampere at 5 volts. I have two MK3S and it has been working great for 2 years.