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.
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
| Letter | Meaning |
|---|---|
| R | Resistor |
| C | Capacitor |
| L | Inductor |
| D | Diode |
| LED | Light-emitting diode |
| Q | Transistor |
| SW | Switch |
| IC | Integrated circuit |
Representing Integrated Circuits in a Schematic Diagram
One important symbol that isn’t shown in Table 5-1 is the symbol for an IC (integrated circuit). ICs are small assemblies that usually have multiple leads, called pins, which connect to various parts of the circuit contained within the assembly. Some ICs have as few as six or eight pins; others have dozens or even hundreds. These pins are numbered, beginning with pin 1. Each pin in an IC has a distinct purpose, so connecting to the correct pins in your circuit is vital to the circuit’s proper operation. If you connect to the wrong pins, your circuit won’t work, and you may damage the IC.
The most common way to depict an integrated circuit in a schematic diagram is as a simple rectangle with leads coming out of it to depict the various pins. The arrangement of the pins in the schematic diagram doesn’t necessarily correspond to the physical arrangement of pins on the IC itself. Instead, the pins are positioned to provide for the simplest circuit paths in the diagram. The pins in the diagram are numbered to indicate the correct pin to use.
For example, Figure 5-8 shows a schematic diagram that uses a popular IC called a 555 timer IC to make an LED flash. The 555 has eight pins, and you can see that the schematic calls for connections on all eight. However, the pins in the diagram are arranged in a manner that simplifies the connections to be made to the pins. In an actual 555 IC, the pins are arranged in numerical order on either side of the IC, with pins 1 through 4 on one side and pins 5 through 8 on the other side.
FIGURE 5-8: A circuit that uses an integrated circuit.