the other way around. You can tell which side of an LED is which in three ways:
✓ Compare the lengths of the leads. The shorter lead is the cathode (negative side) and the longer lead is the anode (positive side). (See Figure 2-5, left.)
✓ Peek inside the plastic case. The lead attached to the larger piece of metal inside the case is the cathode (negative side); the lead attached to the smaller piece of metal is the anode (positive side). (See Figure 2-5, left.)
✓ Look (or feel) for a flat edge on the plastic case. This flat edge is on the cathode (negative side) of the LED. (See Figure 2-5, right.)
Look at the leads of your LED. Can you tell which one is the shorter lead? Now look inside the case of your LED. (You may need to shine a flashlight on the case to see inside better.) Can you spot the larger and smaller pieces of metal? Finally, run your finger around the bottom edge of the plastic case. Can you feel a flat edge?
Orienting an LED in a circuit
To conduct current and emit light, most LEDs require between 2.0 and 3.4 volts to be applied across the leads. The exact voltage needed depends on the color of the LED. A 9-volt battery is powerful enough to push current through any LED, but a 1.5-volt battery, such as an AA or AAA battery, isn’t strong enough. For this reason, you use a 9-volt battery rather than an AA or AAA battery for your LED circuit.
To limit the current that flows from your 9-volt battery through your LED, you insert a resistor in your circuit. Resistors slow down current, like a kink in a hose slows the flow of water.
Figure 2-6 shows you a variety of resistors. Every resistor has two leads, and it doesn’t matter which way you insert a resistor into a circuit. Current flows either way through a resistor. (Resistors are not semiconductors, so they are not picky.)
Resistors don’t require a minimum voltage like LEDs do (not picky!). Current flows through a resistor even with a tiny voltage applied. The higher the voltage you apply to a resistor, the higher the current that flows through the resistor – up to a point. Too much current can melt a resistor. (Don’t worry. You won’t melt any resistors for the projects in this book – as long as you follow the instructions!)
Understanding resistance
Every resistor has a value known as its resistance (what a surprise). The higher the resistance, the more the resistor restricts current. Resistance is measured in ohms (pronounced “omes”), and the symbol for ohms is Ω (which looks like an upside-down horseshoe and is the Greek letter omega).
Some resistances are measured in kilohms (pronounced “kill omes”), which means thousands of ohms. The symbol for kilohms is kΩ. Other resistances are so large they are measured in megohms (pronounced “meg omes”), which means millions of ohms. The symbol for megohms is MΩ. (You may be familiar with the prefixes, kilo, which means thousands, and mega, which means millions, from your math classes. And you’ve probably heard of measurements such as kilometer, as in the k in a 5k race, and megabyte, as in “My laptop has 4 megabytes of RAM.”)
For your LED flashlight, you need a resistor with a value of 470 Ω. But resistors don’t have their values stamped on their cases, so you need to know how to identify a 470 Ω resistor. You can tell what the resistance of a specific resistor is by looking at the colored bands on its case. Think of the colored bands as a code. The color and position of the bands tell you the value of the resistance.
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