Electronics All-in-One For Dummies. Doug Lowe

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      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 voltage is provided by some means, either by a battery or by some other device such as a solar cell or a power supply plugged into an electrical outlet.

Figure 5-5 shows a schematic diagram for the same circuit that is shown in Figures 5-3 and 5-4, but with voltage source symbols instead of a battery symbol. As you can see, +6 V is required in two places in the circuit: at the resistor and at the second transistor. This circuit is functionally identical to the circuits shown in Figures 5-3 and 5-4.

       Although the circuit shown in Figure 5-5 has a positive voltage source and the ground is negative, this isn’t always the case. You can also use the voltage source symbol to refer to negative voltage. In that case, the ground actually carries positive voltage back to the source.

      FIGURE 5-5: A schematic diagram that uses a common ground to complete the circuit, with voltage source symbols.

In some cases, a circuit may require both positive and negative voltages at different places within the circuit. Remember from Chapter 2 of this minibook that voltages are always measured with respect to two points in a circuit. Thus, voltages are always relative. For example, the positive pole of a AAA battery is +1.5 V relative to the negative pole. At the same time, the negative pole of the battery is –1.5 V relative to the positive pole.

      Now suppose you connect two AAA batteries end to end. Then, the voltage at the positive terminal of the first battery will be +3 V relative to the voltage at the negative terminal of the second battery. But, the voltage at the positive pole of the first battery will be +1.5 V relative to the point between the batteries, and the voltage at the negative pole of the second battery will be –1.5 V relative to the point between the batteries.

Figure 5-6 shows how this arrangement might be drawn in a schematic diagram, with a pair of resistors connected across each battery to the middle point. The diagram on the left shows the batteries and connections to them. The diagram on the right shows the same circuit using ground and voltage source symbols instead.

      FIGURE 5-6: Two equivalent diagrams showing positive and negative voltage sources.

Labeling Components in a Schematic Diagram

A symbol alone is not usually enough information to completely identify an electronic component in a schematic diagram. Further information is usually included with text that’s placed adjacent to the symbol, as shown in Figure 5-7. This additional information usually includes the following:

      FIGURE 5-7: A schematic diagram with parts labeled.

       Reference identifier: Each component is usually labeled with a letter that designates the type of component followed by a number that helps identify each component of the same type. For example, if a circuit has four resistors, the resistors are identified as R1, R2, R3, and R4. The most commonly used letters are shown in Table 5-2.

       Value or part number: For components such as resistors and capacitors, the value is given in ohms (for resistors) and microfarads (for capacitors). Thus, a 470 Ω resistor would have the number 470 next to it, and a 100 μF capacitor would have the number 100 next to it.The letters k and M are used to denote thousands and millions. For example, a 10,000 Ω resistor is identified as 10k in a schematic.Components such as diodes, transistors, and integrated circuits don’t have values; instead, they have manufacturer’s part numbers. Thus, you might find a part number such as 1N4001 (for a diode), 2N2222 (for a transistor), or 555 (for an integrated circuit, IC) next to one of these components.In some cases, the value or part number is omitted from the schematic diagram itself and instead included in a separate parts list that identifies the value or part number of each referenced part that appears in the schematic. Then, to find the value or part number of a particular component, you look up the component by its reference identifier in the parts list.

TABLE 5-2 Commonly Used Reference Identifiers

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