Tech Thursday: LED Me Not Into Temptation

Permalink 06/30/04  

Having seen so many coolcool applications, and even more misapplications of LEDs and other solid state lights, we thought it might be useful to run through some of the benefits and tradeoffs of the technology, as well as some sources for buying them, and tutorials on their installation.

So, lets get started...
LED

LED Parts and Differences
You all probably know that LEDs are pretty different than incandescent bulbs. First of all, LEDs are semiconducting, so they are more like a computer chip than a light bulb. They are made up of three main parts: a semiconducting diode, which creates the light, a pair of wire leads, which supply power, and hold the chip, and a plastic housing, which protects the chip, and focuses the light like a lens. Because the LED is a diode, it acts like a one way valve, allowing electricity to flow only from the cathode lead to the anode lead. The cathode is indicated by a flat cut in the plastic housing on it's side.
Because LEDs are semiconductors, they have a specific voltage at which they operate. In order to control this voltage when multiple devices are used, a resistor is often added in series with the LED. For help finding the value of this resistor, refer to the LED's documentation, or use a calculator. Now that you have a pretty good idea of how to hook up your LEDs, lets look at some of the unique abilities of these guys

Benefits of LEDs
First of all, LEDs are Chromatic, which means that they emit basically one color of light. This property allows them to be mixed to create other colors, like in a Jumbotron, or other display. ColorKinetics Sauce line of novelty lights use the same concept, but fading a red, green, and blue LED together to generate colors of every hue.

Second, since they are diodes, they can be used in circuits as diodes, but mode importantly, they can be made to change color by running current one way or another through them. LEDs in cheap 3-color reader boards are bipolar like this, so for positive current, they are red, negative they are green, and alternating current causes your eye to see orange. Remember that with light, Red + green = Yellow, Green + Blue = Cyan, and Red + Blue = Magenta, and Red + Blue + Green = white. For more info on colors, try this.

LEDs are also energy efficient, since they are converting most of their energy into light, and not heat, like an incandescent bulb. Efficiencies as high as 10 times those of incandescents can be reached in certain applications. White LEDs are not this efficient, since they use a blue source, and a yellow phosphor to create white light (yellow = red + green, so yellow + blue = white), and some light is lost in the conversion. But if energy efficiency is a need, LEDs are a great choice.

Finally, and probably most importantly, LEDs practically never burn out. Most are rated for 100,000 hours or some ridiculous length of time. They are also more durable than bulbs, since there are no filaments to break. This property, combined with their efficiency is the reason for the mass changeover of traffic signals, walk signs, and commercial truck brake lights over the last few years. White light LEDs are still prohibitively expensive for anything other than flashlights or task lighting, but it's probably only a matter of time and economies of scale.

Drawbacks of LEDs
The major sticking point of LEDs is price, especially for white, purple, and blue colors, which are very newly invented, and therefore not produced in the same giant quantities as red and amber lamps. Adding to this, dimming must be done electronically, which can add to the cost. This is the reason why most adjustable brightness bike lights just switch on more LEDs to be brighter, so that they avoid complex circuitry.

Other Resources
We hope that all this talking made you hungry to go out and buy some LEDs and get experimenting on your next avant-garde light. So, if you're in the market for some components, we recommend the following places to start:

The LED Museum - this place has reviews of basically every LED made since 1970 or so, plus tons of links to tutorials and other information. They even give pictures of the beam spread and color. Amazing.

ColorKinetics - sells computer control software and LED modules mostly to interior design applications, but we imagine they'd love to hear a good idea for product design.

American Science and Surplus - these guys are Chicago natives, so they can't be all bad. Their prices are super cheap, but their inventory ranges from sketchy to ridiculous. Worth a shot for inspiration though.

Digikey - If you haven't been here before, it's going to be tough to navigate; they must have about 1.3 quadrillion parts, all searchable online. But, among all those, they definitely stock LEDs of every color and combination, along with control circuits and power supplies. If you're ready to dig in, this is the place to start.

So, with that, happy lighting, and don't hurt yourselves. If you make anything really cool, we'd love to hear about it.
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