Make: Electronics
Page 3
First inspect the fuse very carefully, using a magnifying glass if you have one. You should see a tiny S-shape in the transparent window at the center of the fuse. That S is a thin section of metal that melts easily.
Figure 1-35. When you attach both wires to the fuse, the little S-shaped element inside will melt almost instantly.
Remove the battery that you short-circuited. It is no longer useful for anything, and should be recycled if possible. Put a fresh battery into the battery carrier, connect the fuse as shown in Figure 1-35, and take another look. You should see a break in the center of the S shape, where the metal melted almost instantly. Figure 1-36 shows the fuse before you connected it, and Figure 1-37 depicts a blown fuse. This is how a fuse works: it melts to protect the rest of the circuit. That tiny break inside the fuse stops any more current from flowing.
Figure 1-36. A 3-amp fuse, before its element was melted by a single 1.5-volt battery.
Figure 1-37. The same fuse after being melted by electric current.
Fundamentals
Volt basics
Electrical pressure is measured in volts. The volt is an international unit. A millivolt is 1/1,000 of a volt.
Number of volts
Usually expressed as
Abbreviated as
0.001 volts
1 millivolt
1 mV
0.01 volts
10 millivolts
10 mV
0.1 volts
100 millivolts
100 mV
1 volt
1,000 millivolts
1V
Ampere basics
We measure electrical flow in amperes, or amps. The ampere is an international unit, often referred to as an “amp.” A milliamp is 1/1,000 of an ampere.
Number of amperes
Usually expressed as
Abbreviated as
0.001 amps
1 milliamp
1 mA
0.01 amps
10 milliamps
10 mA
0.1 amps
100 milliamps
100 mA
1 amp
1,000 milliamps
1A
Background
Inventor of the battery
Alessandro Volta (Figure 1-38) was born in Italy in 1745, long before science was broken up into specialties. After studying chemistry (he discovered methane in 1776), he became a professor of physics and became interested in the so-called galvanic response, whereby a frog’s leg will twitch in response to a jolt of static electricity.
Using a wine glass full of salt water, Volta demonstrated that the chemical reaction between two electrodes, one made of copper, the other of zinc, will generate a steady electric current. In 1800, he refined his apparatus by stacking plates of copper and zinc, separated by cardboard soaked in salt and water. This “voltaic pile” was the first electric battery.
Figure 1-38. Alessandro Volta discovered that chemical reactions can create electricity.
Fundamentals
Direct and alternating current
The flow of current that you get from a battery is known as direct current, or DC. Like the flow of water from a faucet, it is a steady stream, in one direction.
The flow of current that you get from the “hot” wire in a power outlet in your home is very different. It changes from positive to negative 60 times each second (in Great Britain and some other nations, 50 times per second). This is known as alternating current, or AC, which is more like the pulsatile flow you get from a power washer.
Alternating current is essential for some purposes, such as cranking up voltage so that electricity can be distributed over long distances. AC is also useful in motors and domestic appliances. The parts of an American power outlet are shown in Figure 1-39. A few other nations, such as Japan, also use American-style outlets.
For most of this book I’m going to be talking about DC, for two reasons: first, most simple electronic circuits are powered with DC, and second, the way it behaves is much easier to understand.
I won’t bother to mention repeatedly that I’m dealing with DC. Just assume that everything is DC unless otherwise noted.
Figure 1-39. This style of power outlet is found in North America, South America, Japan, and some other nations. European outlets look different, but the principle remains the same. Socket A is the “live” side of the outlet, supplying voltage that alternates between positive and negative, relative to socket B, which is called the “neutral” side. If an appliance develops a fault such as an internal loose wire, it should protect you by sinking the voltage through socket C, the ground.
Background
Father of electromagnetism
Born in 1775 in France, André-Marie Ampère (Figure 1-40) was a mathematical prodigy who became a science teacher, despite being largely self-educated in his father’s library. His best-known work was to derive a theory of electromagnetism in 1820, describing the way that an electric current generates a magnetic field. He also built the first instrument to measure the flow of electricity (now known as a galvanometer), and discovered the element fluorine.
Figure 1-40. Andre-Marie Ampere found that an electric current running through a wire creates a magnetic field around it. He used this principle to make the first reliable measurements of what came to be known as amperage.
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 v
alues 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 th
e 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.
Swap out the 1K resistor and substitute a 470Ω resistor, which will have yellow-violet-brown stripes, meaning 4-7 and one more zero. The LED should be brighter still.
This may seem very elementary, but it makes an important point. The resistor blocks a percentage of the voltage in the circuit. Think of it as being like a kink or constriction in a flexible hose. A higher-value resistor blocks more voltage, leaving less for the LED.
Figure 1-45. Here’s how it actually looks, using a large LED. If you start with the highest value resistor, the LED will glow very dimly as you complete the circuit. The resistor drops most of the voltage, leaving the LED with insufficient current to make it shine brightly.