One of the most common uses for resistors is to limit the current flowing through an electronic component. Some components, such as light-emitting diodes, are very sensitive to current. A few milliamps of current is enough to make an LED glow; a few hundred milliamps is enough to destroy the LED.
Project 2-1 shows you how to build a simple circuit that demonstrates how a resistor can be used to limit current to an LED.
Before getting into the construction of the circuit, here's a simple question: Why a 120 Ω resistor? Why not a larger or a smaller value? In other words, how do you determine what size resistor to use in a circuit like this?
The answer is simple: Ohm's law, which can easily tell you what size resistor to use, but you must first know the voltage and current. In this case, the voltage is easy to figure out: You know that two AA batteries provide 3 V.
To figure out the current, you just need to decide how much current is acceptable for your circuit. The technical specifications of the LED tell you how much current the LED can handle. In the case of a standard 5 mm red LED (the kind you can buy at RadioShack for about $1.50), the maximum allowable current is 28 mA.
To be safe and make sure that you don't destroy the LED with too much current, round the maximum current down to 25 mA.
To calculate the desired resistance, you divide the voltage (3 V) by the current (0.025 A). The result is 120 Ω.
Do not connect the LED directly to the battery without a resistor. If you do, the LED will flash brightly, and then it will be dead forever.