Chapter 3. A Brief Introduction to LEDs

Before we start working on the LED photometer, we’re going to take a brief look at why and how the gadget’s crucial component—the light emitting diode, or LED—can be used to monitor certain properties of the atmosphere.

Even if you’ve worked with LEDs before, chances are your projects used them simply as lights or indicators of some sort, with the color choices more about aesthetics than science. By comparison, in this book we use LEDs in ways that take advantage of the physics of light.

We think you’ll get more out of building and using the LED photometer if you know more about that. But if you’re feeling impatient, skip to the project in Chapter 4, the LED Sensitivity Tester, and proceed from there.


It is essential to test your LEDs before you build the LED photometer. Testing takes two forms: (1) testing to see if the LEDs light up and (2) testing the LEDs for wavelength sensitivity. The first test is easy: you can connect the short wire of an LED to Arduino GND and the long end to pin 13, and run the BLINK sketch that comes with the Arduino IDE software; if the LED blinks, it works. (An even easier way is to connect an LED to a 3 v coin cell—with the long wire on the positive side of the battery. If the LED lights, it works.)

Testing the LED for wavelength sensitivity is much more complicated; we deal with that in Chapter 4.

What Is a Diode?

A diode is a two-terminal electronic device that lets electricity flow easily in one direction, and ...

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