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.
Note
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.
A diode is a two-terminal electronic device that lets electricity flow easily in one direction, and prevents (or resists) it from flowing in the other direction. Many electrical components can be run "backwards”: for example, a loudspeaker can be used as a microphone, and vice versa. A motor, which turns electricity into movement, can often be used as a generator that turns movement into electricity.
This reversibility of electronic components is partly thanks to the fact that electricity usually has no problem flowing both ways through a circuit; it makes no difference to an electron if it flows from a battery to a motor or from a motor to a battery.
But the direction of electron flow matters if we’re designing a generator circuit: we don’t want that circuit to suddenly work in reverse and start spinning like a motor! This is where diodes are useful. They work like valves that prevent the “backflow” of electricity, ensuring that electrons will move only in the direction we want them to.
A light emitting diode is a diode that emits light.
Okay, snark aside, in 1907 a scientist named Henry Joseph Round discovered that crystals of carborundum gave off a faint yellow light when they were energized by electrons. These crystals were also diodes, and didn’t emit light when electricity tried to flow the other way. Scientists later discovered that in an LED, electrons combine with electron “holes” in the crystal, and release excess energy as photons in a very narrow range of wavelengths known as the emission band. The narrowness of this emission band is rivaled only by laser light, and explains the jewel-like purity of LED light.
Figure 3-1 shows some LEDs in sizes 8 mm, 5 mm, and 3 mm. Two important things to know for DIY purposes are that the longer lead is called the anode (commonly thought of as the positive terminal), and the shorter lead is called the cathode (commonly thought of as the negative terminal). An easy way to remember this is that the negative terminal has something “subtracted” from it. Because an LED is a diode, electricity will only easily flow through it in one direction. So when you plug an LED into a circuit, be sure to connect the anode to the positive side of the circuit and the cathode to the negative side or ground, if you expect to make it glow.
LEDs not only emit light; they absorb it, too. An amateur scientist discovered this capability of LEDs in the 1970s. Forrest M. Mims III, a former US Air Force officer, had been doing experiments with photocells and light since he was a boy. Understanding that many electronic devices can be run “backwards” [see What Is a Diode?], Mims reasoned that since LEDs take in electricity and emit light (along that narrow emission band, remember), they should also take in light and give off electricity. A series of experiments proved him right, and by 1973 he had formalized what is now known as the Mims Effect: LEDs will absorb light along a relatively narrow band of color, and emit a small amount of electricity. These light-absorbing qualities of LEDs are what we put to work in our LED photometer.
An LED absorbs light occurring in wavelengths near its own emission band, or slightly shorter. A green LED absorbs light in the greenish-blue part of the spectrum, a red LED absorbs light in the reddish-orange part of the spectrum, and so on. We can use this difference in the way LEDs absorb light to tell us a great deal about the atmosphere.