LED has two terminals: the anode (labeled with plus-sign (+)) and the cathode (labeled with minus -sign (-)). For correct operation, an LED only light when voltage is applied in the forward direction of the diode.
Simple LED circuit with resistor for current limiting
The current in an LED rises exponentially with the applied voltage, so current through the LED must be regulated by an external circuit such as a resistor to prevent damage.
For example, the rating current of LED is 20mA, user have to regulate the current through LED under this rating current. (Please refer to the page of RESISTOR for the detailed explanation and calculation.)
DIP LED (figure from internet) : In addition to different LED light color (eg red, yellow, green, white, blue), user also can find normal-brightness and high-brightness LED on the market. The terminal of DIP LED can be easily identified by the length of lead, long lead is anode and short lead is cathode.
SMD LED (figure from internet) : the size of SMD LED includes 0402, 0603, 0805, 1206 (the larger digit, the large size) The terminal of SMD LED can be identified by a dot on package. Normally dot indicates cathode, but there are exception. Please check the polarity by multi-meter before assembly. (Please refer to the page of MULTIMETER for the details.)