Abstract

What's your favorite thing to do on the hottest day of the year? Dip your toes in an icy river? Hang out by the pool? Retreat to a cool basement? Lie motionless in the shade? You're probably not too eager to move around and put out a lot of energy, like mowing the lawn in the mid-afternoon sun. Well, you're not the only one. In this electronics science fair project, you'll find out that some semiconductor devices, like light-emitting diodes (or LEDs), act the same way. As their internal temperature goes up, their light output goes down.

Objective

To determine how the output of an LED flashlight changes over time as its temperature increases.

Introduction

If you've ever gone camping deep in a forest, far from any town or city, you know well the comfort and security that a small bit of light can bring. Humans learned how to control fire, by some accounts, around 400,000 years ago, but the first portable electric lights didn't come about until the invention of the battery and the incandescent lightbulb, so were not available until 1898. These early portable lights (shown in Figure 1) were first tried out by the New York City police department. They ran on zinc-carbon batteries, which did not provide a good flow of current to the inefficient lightbulbs that were in use at the time. The combination of poor batteries and inefficient bulbs meant that the portable lights could only be turned on for brief periods of time, and then they had to "rest" before they could be used again. For this reason, they were named flashlights, because you could only use them to get a "flash of light" before having to turn them off again. Flashlight technology has improved considerably over the last 100 years, but the name still remains, at least in the United States. In much of the rest of the world, they are called torches.

Traditional flashlights work by flipping the flashlight's on-off switch to the "on" position, which "closes the loop" and allows current to be drawn from the flashlight battery and pass through an extremely thin tungsten filament inside the flashlight's incandescent lightbulb. As free electrons pass through the filament, they bump into and vibrate atoms in the tungsten filament and heat them up. The heat raises bound electrons in the vibrating tungsten atoms to a higher energy state temporarily, and when they fall back down to their normal state, they give off that energy in the form of photons, the basic units of light. The light coming from the bulb spreads out in all directions, but a parabolic reflector collects and focuses the light into a narrow beam, so you can find your way to your tent in the dark.

In the past decade, the traditional flashlight has been modified to use solid-state electronics. The small incandescent lightbulb has been replaced with a semiconductor device, called a light-emitting diode or LED. Semiconductors are called "semi" conductors because they can conduct or carry electricity, but not as well or easily as a normal conductor, like copper wire, can. An LED does not have a filament inside it, like a lightbulb does. Instead, it has a diode containing one semiconductor material with extra electrons (called n-type) bonded together with another semiconductor material with extra "holes," or a deficit of electrons (called p-type). With this arrangement, current can only flow in one direction across the diode.

When a battery with enough voltage is connected across a diode in the proper direction—with the negative terminal connected to the n-type material, and the positive terminal connected to the p-type material—the free electrons in the n-type material are repelled by the negative charge and attracted to the positive electrode, while the holes go the other way, and current flows across the diode. The free electrons are at a higher energy state than the holes are, and when they "fall" into the holes, they release energy in the form of photons, the basic units of light. Whether that light is visible or not depends on how far they fall. LEDs are designed so that the electron falls produce light in the visible spectrum. The greater the fall, the more energy that is released, and the higher the frequency of the light will be.

The color red, for example, has a lower frequency than violet, so the electron "fall" required to produce the color red is shorter than the fall required to produce the color violet.

If you go to a frequency even lower than the visible color red, you'll enter the infrared radiation range. You cannot see infrared radiation, but you can feel it in the form of heat when you get close to a fire, an oven, or an incandescent lightbulb. In fact, most of the energy used in turning on a lightbulb goes toward generating (unwanted) infrared radiation—only 5 percent goes toward producing visible light. In contrast, LEDs feel relatively cool as you get close to them because, in general, they emit very little infrared radiation. They are more efficient than incandescent lightbulbs, meaning that their light output per unit power input (a ratio) is greater. LED efficiency varies widely and depends on things like an LED's color, how it was manufactured, and the amount of current passing through it. No matter what their efficiency, LEDs do radiate heat at their base though, at many other frequencies other than infrared, with the result being that some portion of the input energy goes toward producing visible light, and the rest is spent generating heat.

It turns out that it is important to remove this heat through thermal-management methods, like heat sinks, especially with high-power LEDs, because as the temperature increases, an LEDs efficiency and brightness decrease. In this electronics science fair project, you are going to investigate how the output of an LED flashlight changes over time, after you turn it on and it begins to heat up and approach its steady-state temperature, the point at which its internal temperature is no longer changing and the LED has reached its thermal equilibrium.

Terms and Concepts

• Battery
• Incandescent lightbulb
• Zinc-carbon battery
• Current
• Tungsten filament
• Free electrons
• Bound electrons
• Energy state
• Photon
• Parabolic reflector
• Electronic
• Semiconductor
• Light-emitting diode (LED)
• Diode
• N-type
• P-type
• Infrared radiation
• Efficiency
• Heat sink
• Steady state
• Thermal equilibrium
• Illuminance
• Inverse square law

Questions

• How is light created in a traditional flashlight?
• How is a diode made?
• Can current flow in both directions in a diode?
• How is light created in an LED flashlight?
• Why do LEDs feel cooler when you get close to them than incandescent lightbulbs?

Materials and Equipment

• Shoebox or other small cardboard box
• Scissors
• Ruler
• Multi-color LED flashlight, ideally with an on-off switch on the end of the handle; available at some hardware, sporting goods, and electronics stores, or from online sellers, such as www.amazon.com
• Lux meter, also known as a light meter; such as the Light Meter LX1010B,50,000 Lux Luxmeter with lcd display, available at Amazon.com
• Tape, any sturdy packaging or masking tape
• Timer
• Helper
• Lab notebook

Experimental Procedure

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Electricity & Electronics Project Ideas
• My Favorites

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