Make: Electronics (7 page)
Authors: Charles Platt
Cleanup and Recycling
The first AA battery that you shorted out is probably damaged beyond repair. You should dispose of it. Putting batteries in the trash is not a great idea, because they contain heavy metals that should be kept out of the ecosystem. Your state or town may include batteries in a local recycling scheme. (California requires that almost all batteries be recycled.) You’ll have to check your local regulations for details.
The blown fuse is of no further use, and can be thrown away.
The second battery, which was protected by the fuse, should still be OK. The battery holder also can be reused later.
Experiment 3: Your First Circuit
Now it’s time to make electricity do something that’s at least slightly useful. For this purpose, you’ll use components known as resistors, and a light-emitting diode, or LED.
You will need:
- 1.5-volt AA batteries. Quantity: 4.
- Four-battery holder. Quantity: 1.
- Resistors: 470Ω, 1K, and either 2K or 2.2K (the 2.2K value happens to be more common than 2K, but either will do in this experiment). Quantity: 1 of each resistor.
- An LED, any type. Quantity: 1.
- Alligator clips. Quantity: 3.
Setup
It’s time to get acquainted with the most fundamental component we’ll be using in electronic circuits: the humble resistor. As its name implies, it resists the flow of electricity. As you might expect, the value is measured in ohms.
If you bought a bargain-basement assortment package of resistors, you may find nothing that tells you their values. That’s OK; we can find out easily enough. In fact, even if they are clearly labeled, I want you to check their values yourself. You can do it in two ways:
- Use your multimeter. This is excellent practice in learning to interpret the numbers that it displays.
- Learn the color codes that are printed on most resistors. See the following section, “Fundamentals: Decoding resistors,” for instructions.
After you check them, it’s a good idea to sort them into labeled compartments in a little plastic parts box. Personally, I like the boxes sold at the Michaels chain of crafts stores, but you can find them from many sources.
Fundamentals
Decoding resistors
Some resistors have their value clearly stated on them in microscopic print that you can read with a magnifying glass. Most, however, are color-coded with stripes. The code works like this: first, ignore the color of the body of the resistor. Second, look for a silver or gold stripe. If you find it, turn the resistor so that the stripe is on the righthand side. Silver means that the value of the resistor is accurate within 10%, while gold means that the value is accurate within 5%. If you don’t find a silver or gold stripe, turn the resistor so that the stripes are clustered at the left end. You should now find yourself looking at three colored stripes on the left. Some resistors have more stripes, but we’ll deal with those in a moment. See Figures 1-41 and 1-42.
Figure 1-41.
Some modern resistors have their values printed on them, although you may need a magnifier to read them. This 15K resistor is less than half an inch long.
Figure 1-42.
From top to bottom, these resistor values are 56,000 ohms (56K), 5,600 ohms (5.6K), and 560 ohms. The size tells you how much power the resistor can handle; it has nothing to do with the resistance. The smaller components are rated at 1/4 watt; the larger one in the center can handle 1 watt of power.
Starting from the left, the first and second stripes are coded according to this table:
Black | 0 |
Brown | 1 |
Red | 2 |
Orange | 3 |
Yellow | 4 |
Green | 5 |
Blue | 6 |
Violet | 7 |
Gray | 8 |
White | 9 |
The third stripe has a different meaning: It tells you how many zeros to add, like this:
Black | - | No zeros |
Brown | 0 | 1 zero |
Red | 00 | 2 zeros |
Orange | 000 | 3 zeros |
Yellow | 0000 | 4 zeros |
Green | 00000 | 5 zeros |
Blue | 000000 | 6 zeros |
Violet | 0000000 | 7 zeros |
Gray | 00000000 | 8 zeros |
White | 000000000 | 9 zeros |
Fundamentals
Decoding resistors (continued)
Note that the color-coding is consistent, so that green, for instance, means either a value of 5 (for the first two stripes) or 5 zeros (for the third stripe). Also, the sequence of colors is the same as their sequence in a rainbow.
So, a resistor colored brown-red-green would have a value of 1-2 and five zeros, making 1,200,000 ohms, or 1.2MΩ. A resistor colored orange-orange-orange would have a value of 3-3 and three zeros, making 33,000 ohms, or 33KΩ. A resistor colored brown-black-red would have a value of 1-0 and two additional zeros, or 1KΩ. Figure 1-43 shows some other examples.
Figure 1-43.
To read the value of a resistor, first turn it so that the silver or gold stripe is on the right, or the other stripes are clustered on the left. From top to bottom: The first resistor has a value of 1-2 and five zeros, or 1,200,000, which is 1.2MΩ. The second is 5-6 and one zero, or 560Ω. The third is 4-7 and two zeros, or 4,700, which is 4.7KΩ. The last is 6-5-1 and two zeros, or 65,100Ω, which is 65.1KΩ.
If you run across a resistor with four stripes instead of three, the first
three
stripes are digits and the
fourth
stripe is the number of zeros. The third numeric stripe allows the resistor to be calibrated to a finer tolerance.
Confusing? Absolutely. That’s why it’s easier to use your meter to check the values. Just be aware that the meter reading may be slightly different from the claimed value of the resistor. This can happen because your meter isn’t absolutely accurate, or because the resistor is not absolutely accurate, or both. As long as you’re within 5% of the claimed value, it doesn’t matter for our purposes.
Lighting an LED
Now take a look at one of your LEDs. An old-fashioned lightbulb wastes a lot of power by converting it into heat. LEDs are much smarter: they convert almost all their power into light, and they last almost indefinitely—as long as you treat them right!
An LED is quite fussy about the amount of power it gets, and the way it gets it. Always follow these rules:
- The
longer
wire protruding from the LED must receive a
more positive
voltage than the shorter wire. - The voltage difference between the long wire and the short wire must not exceed the limit stated by the manufacturer.
- The current passing through the LED must not exceed the limit stated by the manufacturer.
What happens if you break these rules? Well, we’re going to find out!
Make sure you are using fresh batteries. You can check by setting your multimeter to measure volts DC, and touching the probes to the terminals of each battery. You should find that each of them generates a pressure of at least 1.5 volts. If they read slightly higher than this, it’s normal. A battery starts out above its rated voltage, and delivers progressively less as you use it. Batteries also lose some voltage while they are sitting on the shelf doing nothing.
Load your battery holder (taking care that the batteries are the right way around, with the negative ends pressing against the springs in the carrier). Use your meter to check the voltage on the wires coming out of the battery carrier. You should have at least 6 volts.
Now select a 2KΩ resistor. Remember, “2KΩ” means “2,000 ohms.” If it has colored stripes, they should be red-black-red, meaning 2-0 and two more zeros. Because 2.2K resistors are more common than 2K resistors, you can substitute one of them if necessary. It will be colored red-red-red.
Wire it into the circuit as shown in Figures 1-44 and 1-45, making the connections with alligator clips. You should see the LED glow very dimly.
Now swap out your 2K resistor and substitute a 1K resistor, which will have brown-black-red stripes, meaning 1-0 and two more zeros. The LED should glow more brightly.
Figure 1-44.
The setup for
Experiment 3
, showing resistors of 470Ω, 1KΩ, and 2KΩ. Apply alligator clips where shown, to make a secure contact, and try each of the resistors one at a time at the same point in the circuit, while watching the LED.