Light emitting diode
To make the LED light up, you need a power supply (any voltage) and a resistor.
The resistance required is as follows: R = (U_power - U_led) / I_max
Example: a LED with voltage drop of Uled=1.2V and a max current of I_max=0,020A (20mA) to be powered by a 5V DC source needs:
R = (5-1.2)/.02 = 190 ohm
You then connect a 190 ohm resistance in series with the LED to make it work.
A few people use LEDs in an unusual way: as sensors.
LEDs used normally: to emit light:
LEDs used "in reverse": as light sensors: