ALLNET Brick'R'knowledge LED blue
The blue LED (Light Emitting Diode) is protected against overload with a series resistor. It only lights up when the diode is connected in the direction of flow, i.e. the current flows from the anode to the cathode. LEDs require a minimum voltage in the direction of flow in order to light up. For blue LEDs it is approx. 3.3V.