Zeroplus Arduino starter kit with Logic Analyzer Manual page 55

REMARKS
1. LEDs are polarized. In other words there is only one direction
in which you can connect them, as opposed to non-polarized
components which are symmetrical. The longest lead is called the
anode (+) and the shortest lead the cathode (-). Sometimes the
leads have been cut and for that reason they should not always
be trusted. Look for the flat edge, it indicates the cathode.
As you can see in this circuit, the cathode is connected to
the ground (-) while the anode is connected to the signal (+).
Reverse it and the current will not be able to flow.
2. It is considered a good practice to add a resistor in series
with an LED to limit the current flowing through it. The higher
the value of the resistor, the dimmer the light. If you remove
the resistor the LED will be extremely bright for a second
before it stops working.
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