The abbreviation LED stands for Light Emitting Diode. LEDs consume less power than light bulbs, and have a much longer life time: about 100000 hours. A regular LED needs a current flow of 10...20mA, and has a forward voltage drop of 1.5 to 2 volts, depending on the color.

With the circuit below, you can test and experiment with LEDs.

Question: What will be a good value for R1? Assume that the voltage across LED D1 is 2 volts, and we want a current flow of 15mA.

Answer: The voltage across R1 will be 9V-2V = 7V. The current flow in R1 will also be 15mA. So R1 should have a value of 7V/15mA = 467Ω. From the E12 series, 470Ω is a good choice.