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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
Download (direct link): electronicsfordummies2005.pdf
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Resistors (R)
Resistors may be the most common component of any electronic circuit. Resistors can be either fixed or variable. In a fixed resistor, the resistance never varies. In a variable resistor, the resistance can be changed. What effects the change depends on the construction of the variable resistor. In some cases you manually effect the change, for example, by turning a knob; or the change can be caused by an outside stimulus, such as a change in light, voltage, or temperature. See the section “One Size Fits All: Adjustable Components,” later in this chapter for more about variable resistors.
Transistors (Q)
You often use a transistor in circuits to function either as a switch or as an amplifier. Most transistors have three wires (sometimes four). The arrows in the symbol indicates the type of transistor. For example, in a bipolar PNP transistor type, the arrow faces the base. In a bipolar NPN transistor type, the arrow faces away from the base. (To catch up on the parts of a transistor, such as the base, you can review Chapter 4).
Bipolar transistors are among the most common transistors, but you can also run into other transistor types, such as the field-effect transistor (FET) and the unijunction transistor (UJT). Also note that there are light-sensitive transistors that switch on when exposed to light. You can see symbols for PNP, NPN, and FET transistors types in Figure 6-5.
Transformers (T)
Transformers do just what their name implies: They transform an electric current and voltage to either a higher or lower value. You commonly find transformers in two sections of a circuit:
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Chapter 6: Reading a Schematic 133
Power supply section: Where you use the transformer to step down the 117 VAC line voltage to a lower level, such as 12 or 18 volts
^ Audio output section: To change the impedance (the measure of opposition in an electrical circuit to a flow of alternating current) of the circuit to a level suitable for driving an audio speaker
Figure 6-5:
Variations on a theme.
These symbols represent various types of transistors.
Logic gate symbols
Schematic diagrams for many digital circuits use logic gate symbols. Logic gate symbols indicate the action that occurs in response to the two possible voltage states (on or off). Other than power, these voltage states are the only two present in a digital circuit. You can see the most common logic symbols in Table 6-1.
If logic gates are a mystery to you, go to Chapter 5 to study up on what they are, how they work, and their possible states of on/off or high/low.
Table 6-1 Common Logic Gate Symbols
Name Symbol Function
AND > Output is binary 1 only if both inputs are binary 1
NAND > Same as AND, but output is inverted (binary 0)
OR Î Output is binary 1 if either input is binary 1
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134 Part III: Putting It On Paper
Table 6-1 (continued)
Name Symbol Function
NOR Same as OR, but output is inverted (binary 0)
Buffer Provides a protective buffer or additional drive current between two circuits
Inverter Similar to Buffer, but the output is inverted
Flip flop > Output toggles between 0 and 1
Although you can create AND, OR, and other digital logic gates with transistors, most circuits use an integrated circuit chip (called an IC). One IC contains a number of individual logic gates. For example, the 7400 integrated circuit contains four gates sharing a single power connection.
Some schematics show individual logic gates, and some show connections to the full integrated circuit. You can see an example of each in Figure 6-6. Whether the schematic uses individual gates or an entire IC package, it usually notes the power connections. When it doesn’t, you have no choice but to look up the so-called pinout of the device in a reference book. The pinout is a reference sheet that indicates what each of the connections, or pins, of the integrated circuit is used for. You can often find pinout diagrams on data sheets that manufacturers of integrated circuits provide. You can locate them on the Web using your favorite search engine.
Miscellaneous symbols
You may run across several miscellaneous symbols used in schematics to represent various kinds of electronic gear. For the most part, these symbols are self-explanatory, so we keep things simple and to the point in this section.
However, take special note of the symbols used for switches. The schematic symbol for the switch indicates the number of poles (connections) and positions in the switch. Each pole can switch a different part of the circuit, such as a portion of a circuit that requires a different voltage. (Switches are covered in more detail in Chapter 7).
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Chapter 6: Reading a Schematic 135
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