Introduction: Light Bulb Vs. LED and Limiting Current

The following information is a single lesson in a larger project. Find more great projects here.

Return to Previous Lesson: Prototype with a Breadboard

Lesson Overview:

In this lesson you will learn how to slow down current flow to change the brightness of light bulbs and LEDs.

Step 1: Introduction

In this lesson, we will briefly compare an incandescent light bulb like you find in your house, to an LED that you find in electronics.

You will also measure the current flow through an LED using a new tool in the component library: the multimeter. The multimeter is used to measure multiple aspects of a circuit including resistance, current, and voltage.

  1. Continue to the next step.

Step 2: Incandescent Light

Up until now, you have been using an incandescent light bulb in your circuits. In this lesson you will learn the difference between this type of bulb and an LED, or “light emitting diode.”

You’ve probably seen some incandescent light bulbs around your house. A light bulb is a simple device that converts electrical energy to heat and light by running current through a thin thread of metal called a “filament.” If you have used this type of bulb, you have probably noticed that it gets very hot in addition to producing light!

  1. Continue to the next step.

Step 3: The LED

LEDs, or light emitting diodes, are used a lot in electronic devices. They are also gradually replacing overhead lighting in our homes and offices! LEDs have a lot of special properties that incandescent lights don’t have.

First, LEDs use electrical energy a lot more efficiently than incandescent lights. The only produce light without wasting energy heating up!

Another special property is that they are “polarized” meaning that they have a (+) and (-) side, just like the battery. The “D” in LED stands for diode. What this means is that it will only allow electrical current to flow in one direction. If you connect it the wrong way, it will not light up! In fact, it will block the flow of current.

Finally, LEDs come in a large variety of sizes, colors, and shapes.

In the next step, we will add an LED to the breadboard.

The image below shows some of the many shapes of an LED.

  1. Continue to the next step.

Step 4: Making an LED Circuit (part 1)

Check out the schematic symbol for the LED. The symbol gives some clues about what the component does.

First, the symbol is shaped like an arrow. The direction of current flow is the direction that the arrow is facing. In your circuit, the arrow points away from the positive end of the battery and towards the negative terminal.

Second, the symbol has arrows pointing way from it – this represents light being produced! We colored the arrows yellow for the illustration.

Follow the instructions below to create an LED circuit. A 9V battery and voltage rails are already connected.

  1. Move the LED in the middle of the breadboard in sockets F25 and F26.
  2. Connect a wire between a breadboard hole under the left leg of the LED (- or “cathode”) with the blue voltage rail at the bottom.
  3. Connect a hole under the right leg of the LED (+ or “anode”) with the red voltage rail at the bottom.
  4. Simulate the circuit. What happens?
  5. Press the "next" button below to continue.

Step 5: Making an LED Circuit (part 2)

Notice how the LED looks like it’s exploding? That’s because 9V batteries provide too much current for the LED. If you did this with a physical LED and battery, the 9V battery would destroy the LED in a puff of smoke!

Next you will learn how to protect the LED by limiting the amount of current running through it - with a resistor.

  1. Continue to the next step.

Step 6: Adding a Resistor

LEDs are sensitive to the amount of current running through them. As you just saw, if it’s too high, you can actually damage the LED. In the case of the LED, a small current can provide enough energy to light up the bulb, but not explode it!

A resistor is a conductive material, like a wire, but it’s a bad one. Current travels through a resistor more slowly. In our car illustration, if a wire is a 4 lane-highway, a resistor is like a one-lane winding dirt road.

Even the component symbol for a resistor looks like a zig-zagging road!

Whenever you use an LED in a circuit, it is a good idea to add a resistor IN SERIES, meaning that they share a connection at one terminal. Current flows through the LED and resistor in sequence. You can think of the LED plus resistor as a unit: a protected LED.

  1. Delete the right side wire that connects the LED to the + rail
  2. Click the Components+ and scroll to find the Resistor. Drag the resistor to the work area
  3. Place the resistor where the wire used to be. This will connect the red (+) rail to the right side of the LED through the resistor.
  4. Click the “Start Simulation” button. The LED will light up!
  5. Press the "next" button below to continue.

Step 7: Changing Resistor Values (part 1)

While you are simulating the circuit, you can change the value or strength of the resistor.

Higher value resistors will restrict the current more than low-value resistors - this results in dimmer LEDs, slower motors, and softer buzzers.

When you click on the resistor in the Workplane, an inspector window pops up. (See picture.) There are two things that you can change: the value of the resistor and the units. Here's how it works:

- The basic unit of resistance is Ohms and the symbol is the greek letter Omega. If you want to change resistance one Ohm and a time, set the units to Omega and adjust the number up and down.

Example: 500 Ohms

  1. Highlight the resistor in the Workplane to view its inspector window.
  2. Continue to the next step for more information

Step 8: Changing Resistor Values (part 2)

Notice that the next units in the drop-down window are K-Omega.

- The units of Kilohms (Kilo-Ohms, K-Omega) increase the value of the resistor by a factor of 1000.

For example, 1 Kilohm = 1000 Ohms

10 Kilohms = 10,000 Ohms

- The units of Miliohms (m-Omega) reduce the value of the resistor by a factor of 1000.

For example, 1 miliohm = 1/1000th Ohm or 0.001 Ohms or "one thousandth" of an Ohm.

The pattern continues for the rest of the units:

1 mOhm = 1 thousandth of an Ohm

1 muOhm = 1 millionth of an Ohm

1 nOhm = 1 billionth of an Ohm

1 pOhm = 1 trillionth of an Ohm

1 KOhm = 1 thousand Ohms

1 MOhm = 1 million Ohms

1 GOhm = 1 billion Ohms

  1. Try changing the units of the resistor value. You will make finer adjustments to the value in the next exercise.
  2. How many ways can you express 500 Ohms using a combination of a value and a unit? (see hint)
  3. Press the "next" button below to continue.
  4. Stuck? HINT: 500 Ohms, 0.5 KOhms, 500,000 mOhms are a few examples.

Step 9: Measuring Current Flow (part 1)

Current flowing through a component is measured in Amperes or Amps. A higher current means that more electrons are flowing through the circuit per second!

In this step, we’ll figure out how much current an LED can handle. This will tell us how strong of a resistor we need to reduce the current. You’ll need your Multimeter!

When you use a multimeter to measure current flow, you can think of it as a component in the circuit. Since you measure current through a component, it needs to be IN SERIES with the components that you are measuring the current through.

In this example, all the current runs THROUGH the LED, through the multimeter, and then through the resistor and back into the battery. We’ll start by making some room in the LED circuit.

  1. If the circuit simulation is still running, turn it off by pressing the “Stop Simulation” button
  2. Right now, there isn’t any room in the circuit to add the multimeter. Make some space by selecting the resistor and shifting it to the left by 2 or 3 breadboard sockets.
  3. Add a multimeter to the Workplane using the Components + menu.
  4. Position the multimeter between the LED and resistor using wires, as shown in the picture above.
  5. Simulate the circuit!
  6. Press the “next” button below to continue.

Step 10: Measuring Current Flow (part 2)

Now we can see how the current changes when we swap in different resistor values. Can you figure out the highest current value that the LED can handle?

  1. With the simulation running, select the resistor by clicking on it. A small inspector window will open in the upper left.
  2. Change the value of the resistor using the dropdown menu in the inspector.
  3. While changing the resistance value, up and down, observe the change in current flow.
  4. Find the lowest resistor value that you can use without exploding the LED! When you get close the value, try changing the resistance value 1 ohm at a time.
  5. When you find the lowest possible resistor value, you can check the multimter. This will show the MAXIMUM current that is allowed to flow through the LED. What is the value? (see hint)
  6. Press the "next" button below to continue.
  7. Stuck? HINT: 345 Ohms, 20 mA maximum.

Step 11: What Is the Value of a Resistor?

You have successfully created your first LED breadboard circuit!

In the next lesson we will develop a further understanding of how resistors work and how they act a lot like a network of roads in a circuit neighborhood.

Next Lesson:Slow Down Current with Resistors


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