Adjusting the Brightness of an LED

Guiding Question: How can you adjust the brightness of an LED with a resistor?

Guiding Question: What is the purpose of a resistor?

A resistor in a circuit dissipates the flow of current in the form of heat. Decreasing the flow of current decreases the brightness of the LED. Including a resistor in a circuit can also protect the LED. All LEDs have a maximum amount of current that they can safely handle without overheating. A resistor can be used to limit the amount of current to a safe amount for the LED.




For a given power source, such as a 3 volt battery, there is a safe operating range for an LED. If the value of a resistor in an LED circuit is too high, the LED will not light at all. If the value of the resistor is too low, excessive current may damage the LED. Imagine a water wheel turned by the flow of water in a Roman aqueduct. If the flow of current is too great, the water wheel may be damaged. Placing a baffle in the aqueduct can limit the flow of current and protect the water wheel and protect the circuit. A resistor in an electrical circuit serves the same function.




Edison’s light bulb consisted of a filament in a glass bulb that glowed when an electrical current passed through it. The electrical current also heated the filament, causing it to become hot. Excessive current caused the bulb to burn out. Edison spent months developing mechanisms to regulate the current so that the light bulb would not burn out. A modern LED is more efficient than Edison’s light bulb. However, there are still limits to the amount of current that an LED can handle.




Activity: Exploring Resistors and LEDs

In this project you will explore the effect of different values of resistors on the operation of an LED. Note that current handling capacity of an LED can vary by color, so the results for one color of LED may not be the same as the results for a different value of LED.




A circuit with two AA batteries, a resistor, and an LED


Explore. Try lighting up the LED with a variety of resistor values. What do you notice? Use the following chart or table to record the relationship between the value of a resistor and the brightness of an LED.


Resistance versus Brightness

Resistor Value

Brightness of LED



What is the highest value of resistor for which the LED is still visible?  


What is the effect of the lowest value of resistor on the LED?


What do you think might happen if you use a 9-volt battery instead of the 3-volt AA batteries?



Representing Your Circuit

Representing your circuit

The symbol for a resistor in an electrical circuit is a zigzag line. This indicates the flow of electrical current is impeded by an obstruction. The symbol for a battery and LED are also shown in the table below. The symbol for a battery represents one cell of a battery with the positive terminal shown on one side and the negative terminal shown on the other side. The symbol for an LED consists of an arrow in a circle. The arrow indicates that the electrical current can only flow in that direction when the LED is laced in a circuit.











Schematic symbols for resistor, battery, and LED


The symbols can be depicted in a schematic drawing to illustrate how the components of a circuit are connected. The schematic diagram on the left in the illustration below depicts the same relationships as the pictorial representation on the right. Electrical symbols represent a type of language that engineers use to communicate with one another. Therefore the ability to read and interpret schematic drawing is an important skill.

Create your own schematic. Use the electrical symbols shown above to create a schematic drawing of a circuit that you constructed for your project.