m I learned about a closed circuit, and how to read power supply notification from the back label of it.

m It shows that it transfers the electricity from 120V AC to 12V DC. and I connected the Power connector that can connect to your power supply with a black colored line meaning negative and a red colored line meaning positive in usual electricity. To connect I soldered it.

m I made a closed circuit with a 220 Ω (Ohm) Resistor, a "7805" 5V Regulator and an LED (Light Emitting Diode). And I also connected it with switch for convenience. At my first try I made a mistake by putting LED in a wrong location. From the experience I could make sure that LED's long leg, anode, should be located in positive place, (in case of mine I had to put it on the line of register) and short leg, cathode, should be connected to the ground.

m Then I connected several LEDs in that circuit in different ways. In a circuit, each LED's cathode should be put in ground, but in a diagram of short circuit, the it need to be connected in a line and one direction of it (cathode's direction) is connected to the ground.