Electronics All-in-One For Dummies. Doug Lowe
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a dot to indicate a junction and a hop to indicate a nonjunction. You’ll never see lines simply cross without a hop or a dot.
Looking at Commonly Used Symbols
The circuit shown in Figure 5-1 has just two components: a battery and a lamp. Most electronic circuits will have additional components. There are hundreds of different types of electronic components, and each has its own unique schematic diagram symbol. Fortunately, you need to know only a few basic symbols to get started. These symbols are summarized in Table 5-1. (Note that when used in an actual circuit diagram, the symbols are often rotated.)
Figure 5-3 shows a schematic diagram that includes several of these components. Don’t worry — you don’t need to understand this diagram right now. I just want you to get an idea of what real-world schematic diagrams look like and how to read them.
TABLE 5-1 Common Symbols for Schematic Diagrams
| Symbol | Description |
|---|---|
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Battery |
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Capacitor |
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Diode |
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Ground |
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Inductor (coil) |
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Lamp |
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Light-emitting diode |
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Resistor |
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Source voltage connection |
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Speaker |
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Switch |
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Transformer |
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Transistor (NPN) |
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Transistor (PNP) |
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Variable resistor (potentiometer) |
As you can see, the circuit shown in Figure 5-3 contains six components. Working from left to right, they are:
6 V battery
NPN transistor
Resistor
Capacitor
PNP transistor (at the top right)
Light-emitting diode (at the bottom right)
FIGURE 5-3: A typical schematic diagram.
Throughout the course of this book, I use these and other symbols in the schematic diagrams that describe the circuits. Whenever I use a symbol for the first time, I explain what it is and how it works.
Simplifying Ground and Power Connections
In many electronic circuits, the distribution of voltage connections is one of the most complicated aspects of the circuit. For example, about half of the connections in the schematic diagram shown in Figure 5-3 are used to connect the resistor, transistors, and the LED to either the positive or negative terminal of the battery.
In a more complicated circuit, there can be dozens or even hundreds of power connections. If all the lines representing those connections had to be drawn to the positive or negative side of the battery symbol, schematic diagrams would quickly be overwhelmed by the power connections.
Most circuits have a common path by which current returns to its source. In the case of Figure 5-3, it’s the conductor at the very bottom of the diagram that collects current from the LED and the resistor and returns it to the battery. This conductor is necessary to complete the circuit so that current can flow in a complete loop from the battery through the various components and then back to the battery.
This common return path is often called the ground, and can be replaced by the ground symbol, as shown in Table 5-1. Figure 5-4 shows a schematic diagram that uses three ground symbols to indicate the path by which current returns to the battery. The circuit shown in Figure 5-4 is identical in function to the circuit shown in Figure 5-3.
FIGURE 5-4: A schematic diagram that uses a common ground to complete the circuit.
In addition to a common ground path, most circuits also have a common voltage path. In the case of the circuit shown in Figures 5-3 and 5-4, the common voltage path goes from the battery to the resistor and on to the second transistor. This conductor can be replaced by symbols representing voltage sources that appear wherever voltage is required in a circuit.
The symbol for a voltage source is either an open circle or an arrow. The quantity of voltage is always indicated next to the circle or arrow. When a voltage source symbol is used in a schematic diagram, the symbol for the battery (or other power source if the circuit isn’t powered by a battery) is omitted. Instead, the presence of voltage source symbols implies that