Examining how schematic diagrams provide a road map for electronic circuits
Looking at the most commonly used component symbols
Noting how voltage supply and common ground circuits are often drawn
Seeing how components are typically labeled
I love maps. I think I’ve kept every map I’ve used on every trip I’ve taken. I have big maps of entire countries and states, maps of cities, walking maps, maps of parks and museums, and even subway maps. My favorite maps are topographical maps of the areas where I’ve gone on weeklong backpacking trips. These maps not only show the routes I’ve hiked, but also have elevation lines that represent every painful uphill step I’ve carried my 50-pound backpack up.
Without maps, we’d be lost. We’d never get to our destinations because we wouldn’t know where the roads are. Think of all the sights we’d miss along the way!
Electronics has its own form of maps. They’re called schematic diagrams.
They show how all the different parts that make up an electronic circuit are connected.
Just as maps use symbols to represent features like cities, bridges, and railroads, schematic diagrams use special symbols to represent the different parts of a circuit, such as batteries, resistors, and diodes, and like maps, schematic diagrams have conventions that almost always are used. For example, positive voltages are
almost always shown at the top of a schematic diagram, just as north is almost always shown at the top of a map.
In this chapter, you learn about the symbols used in schematic diagrams and the conventions used to draw them.
Introducing a Simple Schematic Diagram
I’ve read a lot of computer programming books in my day, and I’ve written a few too. In a computer programming book, the first complete computer program usually shown is a program called Hello World, a program that simply displays the text “Hello World!” on a screen, and then quits. It’s pretty much the simplest possible computer program that can be written. It doesn’t do anything useful, but it’s a great starting point for learning how to write computer programs.
Figure 5-1
shows a schematic diagram that is the electronic equivalent of the Hello World program. This diagram is about the simplest schematic diagram possible that actually does something: It lights a lamp, thus announcing to the world that a circuit is indeed working.
FIGURE 5-1:
A simple schematic of a circuit that lights a lamp.
This diagram contains two symbols representing the two components in the circuit: a 1.5 V battery and an incandescent lamp. The lines that connect the two components represent conductors, which could be actual wires or traces of copper in a printed circuit board.
In the circuit depicted in this schematic, the positive side of the battery is connected to one lead from the lamp, and the other lead from the lamp is connected to the negative side of the battery. Once these connections are made, current will
flow from the battery to the lamp, through the lamp’s filament to produce light, and then back to the battery.
Schematic diagrams always depict conventional current flow,
which, as you learn in Chapter 2
of this minibook, means that current flows from positive to negative. Thus, the current flows from the positive terminal of the battery through the lamp and then back to the negative terminal of the battery.
In reality, conventional current flow is opposite of the actual flow of electrons through the circuit. The negative side of the battery has an excess of negatively charged particles (extra electrons) whereas the positive side has an excess of positively charged particles (missing electrons). Thus, the electric charge flows through the conductor from the negative side of the battery, through the lamp, and back to the positive side. (For more about the difference between real current flow and conventional current flow, refer to Chapter 2
of this minibook.)
As it passes through the lamp, the resistance of the lamp’s filament causes the current to heat the filament, which in turn causes the filament to emit visible light.
Laying Out a Circuit
One of the most important things to realize about a schematic diagram is that the arrangement of components in the diagram doesn’t necessarily correspond to the physical arrangement of parts in the circuit when you actually build the circuit.
For example, the circuit depicted in Figure 5-1
shows the battery on the left side of the circuit and the lamp on the right. It also shows the battery oriented so that the positive terminal is at the top and the negative terminal is at the bottom. However, that doesn’t mean the circuit would actually have to be built that way. If you want, you could put the lamp on the left and the battery on the right, or you could put the battery at the top and the lamp on the bottom.
The physical arrangement of the circuit doesn’t matter as long as the component connections remain the same as shown in the schematic. Thus, in this example, no matter how you physically arrange the components, you must connect the positive terminal of the battery to one lead of the lamp and the negative terminal to the other lead.
Because there are only two components and two conductors in the circuit shown in Figure 5-1
, it would be pretty hard to mess up the connections. However, in a more complicated circuit with perhaps dozens of components and dozens of connections, laying out the circuit and making sure that all the connections exactly
match the connections indicated in the schematic can be a challenge. Each connection must be checked carefully to make sure it’s correct.
To Connect or Not to Connect
One of the goals when laying out a schematic circuit diagram is to keep the diagram as simple as possible. However, the lines in all but the simplest of schematic diagrams will at some places need to cross over each other. When they do, it’s vital that you can tell whether the lines that cross represent actual connections (also called junctions
) between the conductors or the lines cross over each other but don’t actually connect.
Unfortunately, there isn’t one clear and universally used standard that dictates how to indicate whether crossed lines represent a junction. Figure 5-2
shows some of the ways for showing crossed wires with or without junctions.
FIGURE 5-2:
Wires that cross may or may not actually be connected.
The three examples on the left side of Figure 5-2
show how junctions are indicated. The example at the top left shows the most common way to indicate a junction: by placing a conspicuous dot at the point where the wires cross. Any time you see a dot where two lines intersect, you know that the two lines form a junction.
In the two junction styles shown in the middle-left and bottom-left examples in Figure 5-2
, the vertical lines are angled to avoid coming together at the same spot on the horizontal line. With or without the dot, junctions are clearly indicated in both of these examples.
The three examples on the right side of Figure 5-2
show how lines that cross but don’t connect to form junctions are most commonly shown. In the top two examples, one line “hops” over the other, and one of the lines is broken at the spot where it crosses the other.
The example in the bottom-right corner of Figure 5-2
is a bit ambiguous. Here, the lines cross each other. However, there’s no hop or break to indicate that no junction is present, nor is there a dot to indicate that a junction should be present. So is there a junction here or not? The answer is, in most cases, no. You can usually assume that a junction is not
present when lines cross but there’s no dot. However, you should examine the rest of the diagram to make sure. If you find other places in the diagram where nonjunctions are indicated by a hop or a break, the crossed lines without the hop or break may indeed indicate a junction.
To avoid ambiguity altogether, the schematic diagrams in this book always use 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 you 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.
As you can see, the circuit depicted 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)
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 that was 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 was shown in Figures 5-3
and 5-4
, but with voltage source symbols instead of a battery symbol. As you can see,
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.
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.
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
relative to the negative pole. At the same time, the negative pole of the battery is
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
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
relative to the point between the batteries, and the voltage at the negative pole of the second battery will be
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
resistor would have the number 470 next to it, and a
capacitor would have the number 100 next to it.
The letters K and M are used to denote thousands and millions. For example, a
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.
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.
Don’t worry about any of the details of the operation of this circuit. You learn how it works in Book 3, Chapter 2
. My only purpose for including it here is so you can see how integrated circuits are depicted in a schematic diagram.
Examining how schematic diagrams provide a road map for electronic circuits
In reality, conventional current flow is opposite of the actual flow of electrons through the circuit. The negative side of the battery has an excess of negatively charged particles (extra electrons) whereas the positive side has an excess of positively charged particles (missing electrons). Thus, the electric charge flows through the conductor from the negative side of the battery, through the lamp, and back to the positive side. (For more about the difference between real current flow and conventional current flow, refer to 
To avoid ambiguity altogether, the schematic diagrams in this book always use 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.
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 
relative to the negative pole. At the same time, the negative pole of the battery is 
relative to the positive pole.
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 
relative to the point between the batteries, and the voltage at the negative pole of the second battery will be 
relative to the point between the batteries.
resistor would have the number 470 next to it, and a 
capacitor would have the number 100 next to it.
resistor is identified as 10K in a schematic.