White LEDs

White LEDs (light emitting diodes) are getting very bright and very cheap. LEDs have the advantage of being very efficient: virtually all of the electrical energy is converted to light, with little heat. (In contrast, a typical incandescent bulb dissipates 95% of its energy as heat, not light.) LEDs also have a very long lifetime (typically rated for over 100,000 hours), and are very rugged.

The big disadvantage of white LEDs is that they have a bluish-yellowish spectrum. They're very "white" but not quite pure white.

Obtaining Parts

The best way to obtain white LEDs for experimentation is to order them directly from Asia through suppliers on eBay (just search for "white leds").

A good device experiment with are the 10mm "JUMBO" devices. In late 2006, I bought 100 LEDs (at 140,000 mcd brightness) for about $40, half of which was the shipping charge. The package arrived in about a week.

You can also buy parts from Radio Shack, All Electronics, DigiKey, and other retail and mail order sources, but you will pay more and not get the latest, brightest devices.

Hooking Them Up

Hooking up the LEDs is easy: any voltage source over 3.6V will do. You can power them from lower voltages (such as a 3V source using two AA cells), but they will not put out full brightness.

Many white LEDs operate at 3.6V with a current of 20 milliamps. If you put them on 3.6V, they should work just fine. If you are powering them from a higher voltage, you need to use a voltage drop resistor in series to limit the voltage to the LED.

Assuming an operating point for the LED of of 3.6V at 20ma, you can calculate the value of the resistor using Ohm's Law.

TBD