Light-emitting diodes

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We’re going to shine some light on some bright bits of high technology: LEDs — light-emitting diodes. Today on Engineering Works!

A few years ago, only scientists and engineers knew about light-emitting diodes. Now you see them everywhere — tiny flashlights, instrument lights, TV remote controls, truck and bus tail-lights, traffic lights. Even stadium TV screens – all lit up by LEDs.

LEDs are one of an array of tiny electronic devices known as semiconductors, made from silicon. Unless you’ve been living in a cave, you already know about transistors and microchips. They’re semiconductors. Normal silicon doesn’t conduct, or carry, electricity. It’s an insulator – a non-conductor. If you add tiny amounts of different stuff to silicon, it starts conducting electricity. Not a lot, but it does conduct it. Different things happen depending on what you added to the silicon. Now you’ve got a semiconductor.

The simplest semiconductors are diodes. Depending on how they’re made, some diodes carry current in one direction. Some in the other. Light-emitting diodes are like other diodes, except that when an electric current hits them, they light up instead of just carrying electricity. But unlike light bulbs, they don’t get hot. And they’re really small, about the size of your little fingernail. And for their size, light-emitting diodes are really bright. This makes them handy for all the things we were just talking about.

It’s time to light up our diode somewhere else. We’ll talk with you later.

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Stop – in the name of engineering

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Today, we’re going to talk about lights — green ones, yellow ones, red ones: traffic signals. On Engineering Works!

Rush hour traffic tie-ups are nothing new. Before we had cars and trucks to make city traffic, we had horses and wagons. They got tied up at busy intersections just as we do today. Imagine being stuck behind the Anheuser-Busch Clydesdales. Whew!

The first traffic light was a red-and-green gas-powered lantern in London in 1868. A policeman turned it from red – stop – to green – caution. Everything worked fine until the day it blew up.

The traffic situation just got worse after the automobile was invented. But the first electric traffic signals didn’t come along until the 1920s. The first one seems to have been invented by Detroit policeman William Potts. Potts borrowed the green-yellow-red sequence of lights we know today from railroad signals. Potts’ first signal was set up at Detroit’s main downtown intersection.

About the time Potts was inventing his traffic light in Detroit, Cleveland, Ohio, businessman Garrett Morgan was working on a traffic signal, too. Morgan’s signal was a t-shaped pole that had three positions — stop, go, and one that stopped traffic from all directions for pedestrians. His was the first to be controlled automatically, and he made it to the patent office first. It was used all over North America until the current green-yellow-red system of electric traffic lights replaced it.

Our light is turning red, so we’ll stop now.

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At the sound of the beep …

At the sound of the beep, please leave your message. Let’s listen – answering machines. Today, on Engineering Works!

It’s hard to get along without an answering machine or voicemail these days. Everybody’s got one. And guess what? Answering machines have been around longer than you think. In fact, telephones were still new when inventor Thomas Edison started thinking about a way to record telephone messages. It was an interesting idea, but he got sidetracked by another strange idea – the phonograph.

The first answering machines took messages from the central telephone directory in the days before direct dialing. And for many years, both telephone companies and the Federal Communications Commission made it almost impossible for folks like you or me to use one. They were expensive, too. You could hire somebody to answer your phone for what you’d pay for an answering machine.

An engineer in Milwaukee, Wis., invented what has to be the most complicated answering machine, in 1949. A mechanical arm picked up the phone when someone called. Then a phonograph in another box played a 78 r.p.m. record with your message to callers into the handset. And then a separate recorder recorded your callers’ message on a piece of wire. Whew!

Today’s answering machines use digital technology to answer your telephone when you’re not there and take messages. No tape or phonograph record involved – pretty slick.

Our message is just about over for this time. Listen again later.

Let’s talk trash

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Listen up. We’re going to talk trash today — plastics — on Engineering Works!

Everybody loves to hate plastic. It’s everywhere these days — fast food containers and foam cups. We throw out 14 million tons of it every year. But plastic is also comfortable contact lenses, disposable syringes in the hospital and the dashboard in your car. But plastic never goes away. When archaeologists dig into landfills thousands of years from now, they’ll probably find that plastic bag you threw out last week.

Chemical engineers are changing that. They’re developing a new kind of plastic that’s made from wheat or corn starch: corn-based plastics, called polylactides, or PLAs.

There’s good news and bad news about PLAs. Unlike the plastic we’re used to, PLAs won’t last forever. They do break down after a while. The sort-of bad news here is that regular landfills won’t do it. You have to put them in special industrial-strength composters. On the other hand, if you burn them, they don’t emit toxic fumes. And it takes a lot less petroleum to make them than regular plastics.

PLA containers aren’t as sturdy as ones made of conventional plastic. They’ll probably melt if you put hot food into them, or if you put them into the dishwasher or microwave. PLAs are still pretty neat. The Coca-Cola company used half a million foam cups made from corn-based PLAs at the 2002 Winter Olympics in Salt Lake City.

We’ve got a way to go before these new plastics will replace the old-fashioned kind. But they’re coming.

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