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What’s the scheme?

You’ll encounter three types of diagrams in electricity and electronics: block, schematic, and pictorial. Each type of diagram serves its own special purpose.

1. A block diagram gives you an overview of how the discrete circuits within a device or system interact. Each circuit is represented with a “block” (a rectangle or other shape, depending on the application). Interconnecting lines, sometimes with arrows on one or both ends, reveal the relationships between the circuits.

2. A schematic diagram (often simply called a schematic) includes every component that a circuit contains, with each component having its own special symbol. This book is devoted mostly to schematics.

3. A pictorial diagram, sometimes called a layout diagram, shows the actual physical arrangement of the circuit elements on the circuit board or chassis, so that you can quickly find and identify components to test or replace.

When you troubleshoot an unfamiliar electronic circuit, you’ll usually start with the block diagram to find where the trouble originates. Then you’ll refer to the schematic diagram (or part of it) to find the faulty component in relation to other components in the circuit. A pictorial diagram can then tell you where the faulty component physically resides, so that you can test it and, if necessary, replace it.

Block diagrams

Block diagrams work well in conjunction with schematics to aid circuit comprehension and to streamline troubleshooting procedures. Each block represents all of the schematic symbols related to that part of the circuit. In addition, each block has a label that describes or names the circuit it represents. However, the block does nothing to explain the actual makeup of the circuit it represents. The blocks play a functional role only; they describe the circuit’s purpose without depicting its actual components. Once you’ve gained a basic understanding of the circuit functions by looking at the block diagram, you can consult the schematic for more details.

To understand how you might use block diagrams, consider the following two examples.

First, suppose that you want to design an electronic device to perform a specific task. You can simplify matters by beginning with a block diagram that shows all of the circuits needed to complete the project. From that point, you can transform each block into a schematic diagram. Eventually, you’ll end up with a complete schematic that replaces all of the blocks.

Alternatively, you can go at the task the other way around. Imagine that you have a complicated schematic, and you want to use it to troubleshoot a device. Because the schematic shows every single component, you might find it difficult to determine which part of the device has the problem. A block diagram can provide a clear understanding of how each part operates in conjunction with the others. Once you’ve found the troublesome area with the help of the block diagram, you can return to the schematic for more details.

Schematic diagrams

A schematic diagram acts, in effect, as a map of an electronic circuit, showing all of the individual components and how they interconnect with one another. According to one popular dictionary, the term schematic means “of or relating to a scheme; diagrammatic.” Therefore, you can call any drawing that depicts a scheme—electronic, electrical, physiological, or whatever—a schematic diagram.

One of the most common schematic diagrams finds a place in almost every car or truck in the United States. It’s a road map, of course, and it portrays a specific sort of scheme. The scheme might involve the paths of travel within a small town, within a state or province, or across multiple states or provinces. Like a schematic diagram of an electronic circuit, the road map shows all the components relevant to the scheme it addresses. Motorists make up their own schemes, which often comprise small portions of the total scheme included in the road map. Likewise, an electronic schematic shows all of the relevant components, and it allows a technician to extrapolate the components and interconnections when testing, troubleshooting, and repairing a small circuit, a large device, or a gigantic system.

Imagine that you want to travel in your automobile from point A to point B. Your road map lists all of the towns and cities that lie between these two points. By comparison, a schematic diagram lists all the components between a similar point A and point B in an electronic circuit. Nevertheless, both of these schematics indicate much more than mere points. You need to know more than which towns or cities lie between two fixed points to get an idea of the overall scheme of things. Indeed, you could easily write down the names of various towns or landmarks, in which case you would not have to resort to a road map at all. From an electronics standpoint, you could do the same thing by compiling a list of the components in a certain circuit, such as:

• 120-ohm resistor

• 1000-ohm resistor

• PNP transistor

• 0.47-microfarad capacitor

• 2 feet of hookup wire

• 1.5-volt battery

• Switch

This list tells us nothing about the circuit in a practical, put-together sense. We know all of the components that we would need if we wanted to build the thing, but we don’t know what it would be if we did! In fact, these components might go together in two or three different ways to make two or three different circuits with different characteristics.

A schematic drawing must indicate not only all components necessary to make a specific scheme, but also how these components interrelate to one another. The road map connects various towns, cities, and other trip components with lines that represent streets and highways. A line that indicates a secondary road differs from a line that represents a four-lane highway. With practice, you can learn to tell immediately which types of lines indicate which types of roads. Likewise, an electronic schematic drawing uses a plain, straight line to indicate a standard conductor; other types of lines represent cables, logical pathways, shielding components, and wireless links. In all cases, when you draw the interconnecting lines, you draw them in order to indicate relationships between the connected components.

Schematic symbology

A schematic diagram reveals the scheme of a system by means of symbology. On a map, the lines that indicate roadways constitute symbols. But of course, a single black line that portrays Route 522 in no way resembles the actual appearance of this highway as we drive on it! We need know only the fact that the line symbolizes Route 522. We can make up the other details in our minds. If people always had to see pictorial drawings of highways on paper road maps, those maps would have to be thousands of times larger than those folded-up things we keep in our vehicle glove compartments, and they would be impossible for anybody to read.

On a decent road map, you’ll find a key to the symbols used. The key shows each symbol and explains in plain language what each one means. If a small airplane drawn on the map indicates an airport and you know this fact, then each time you see the airplane symbol, you’ll know that an airport exists at that particular site, as shown on the map. Symbology depicts a physical object (such as an airport outside a large city) in the form of another physical object (such as an airplane image on a piece of paper).

A good road map contains many different symbols. Each symbol is human engineered to appear logical to the human mind. For instance, when you see a miniature airplane on a road map, you’ll reasonably suppose that this area has something to do with airplanes, so a detailed explanation should not be necessary. If, on the other hand, the map maker used a beer bottle to represent an airport, anyone who failed to read the key would probably think of a saloon or liquor store, not an airport! Because a map needs many different symbols, a good map maker will always take pains to make sure that the symbols make logical sense.

Pure logic will take us only up to a certain point in devising schemes to represent complicated things, especially when we get into the realm of electronic circuits and systems. For example, a circle forms the basis for a transistor symbol, a light-emitting-diode (LED) symbol, a vacuum tube symbol, and an electrical outlet symbol. Additional symbols inside the circle tell us which type of component it actually represents. A transistor is an active device, capable of producing an output signal of higher amplitude than the input signal. We can say the same thing about a vacuum tube, but not about an LED or an electrical outlet.

A circle with electrode symbols inside has been used for many years to represent a vacuum tube. Transistors were developed as active devices to take the places of vacuum tubes, so the schematic symbol for the transistor also started with a circle. Electrode symbols were inserted into this circle as before, but a transistor’s elements differ from a tube’s elements, so the transistor symbol has different markings inside the circle than the tube symbol does. The logic revolves around the circle symbol. Transistors accomplish many of the same functions in electronic circuits as vacuum tubes do (or did), so symbolically they are somewhat similar.

Inconsistencies arise in schematic symbology, and that’s a bugaboo that makes electronics-related diagrams more sophisticated than road maps. A circle can make up a part of an electrical symbol for a device that doesn’t resemble a tube or transistor at all. An LED, for example, can be portrayed as a circle with a diode symbol inside and a couple of arrows outside. An LED is not a transistor or tube, and the electrode symbol at the center clearly reveals this difference. An electrical outlet can serve as another example. It’s absolutely nothing like a tube, transistor, or LED! Yet the basis for the symbol is a circle, just like the circle for a tube or transistor or LED. You’ll learn more about specific schematic symbols in Chap. 3.

Schematic interconnections

To further explore how schematic diagrams are used, let’s consider a single component, a PNP transistor. This device has three electrode elements, and although many different varieties of PNP transistors exist, we draw all their symbols in exactly the same way. We might find a PNP transistor in any one of thousands of different circuits! A good schematic will tell us how the transistor fits into the circuit, what other components work in conjunction with it, and which other circuit elements depend on it for proper operation. A transistor can act as a switch, an amplifier, an oscillator, or an impedance-matching device. A single, specific transistor can serve any one of these purposes. Therefore, if a transistor functions in one circuit as an amplifier, you can’t say that the component will work as an amplifier only, and nothing else. You could pull this particular transistor out of the amplifier circuit and put it into another device to serve as the “heart” of an oscillator.

Suppose that you plan to drive your car from Baltimore, Maryland to Los Angeles, California. Even if you’ve made the trip several times in the past, you probably don’t recall all of the routes that you’ll need to take and all of the towns and cities that you’ll pass along the way. A road map will give you an overall picture of the entire trip. Because all of the trip data exists in a form that you can scan at a glance, the road map plays a critical role in allowing you to see the entire trip rather than each and every segment, one at a time. A schematic diagram does the same thing for a “trip” through an electronic circuit.

Continuing with the road map and the coast-to-coast trip as an example, imagine that you have memorized the entire route from Baltimore to Los Angeles. Assume also that one of the prime highways on the way is under construction, forcing you to take an alternate route. Without a road map, you’ll have no idea as to what detours exist, which alternate route is the best one to take, and which detour constitutes a path that will keep you on course as much as possible and eventually return you to the original travel route with a minimum of delay and inconvenience.

An electronic circuit has many electrical highways and byways. Occasionally, some of these routes break down, making it necessary to seek out the problem and correct it. Even if you can visualize the circuit in your head as it appears in physical existence, you’ll find it impossible to keep in your “mind’s eye” all the different routes that exist, one or more of which could prove defective. When I speak here of visualizing the circuit, I don’t mean the schematic equivalent of the circuit, but the actual components and interconnections, known as the hard wiring.

A schematic diagram gives you an overall picture of a circuit and shows you how the various routes and components interact with other routes and components. When you can see how the overall circuit depends on each individual circuit leg and component, you can diagnose and repair the problem. Without such a view, you’ll have to “shoot in the dark” if you want to get the circuit working again, and you’ll just as likely introduce new trouble as get rid of the original problem!

Visual language

It can prove difficult to explain schematic diagramming in detail to people who have just begun their study of electronics. It helps to think of this form of symbology as a language, that is to say, a system of symbols that helps us to communicate ideas. The English language (or whatever your native language happens to be) is a scheme with a system of symbology with which you’re familiar, of course, because you’re reading this book!

Every word spoken in English or any other verbal language is a complex symbol made from simpler elements called characters. Let’s take the word “stop,” for example. Without a reference key, this sound means nothing. A newborn infant hears noise coming out of your mouth, that’s all! However, through learning the symbology from shortly after birth, this word begins to mean something because the child, who has begun learning to speak and understand, can compare “stop” to other words, and also to actions. We can even say that the word “stop” is a sort of symbology within symbology. The communicator’s intent, when using the word “stop,” can also be expressed by the phrase “Do not proceed further.” This phrase also constitutes symbology, expressing a mental image of a desired action.

If we could all communicate by mental telepathy, then we wouldn’t need language or the symbols that it comprises. Thinking happens a lot faster than speaking or writing or reading can go; and brain processes are the same from human to human, regardless of what language any particular individual employs when speaking, writing, or reading. A newborn baby speaks and understands no language whatsoever. However, whether that baby was born in the United States, South Africa, Asia, or wherever, thought processes take place.

The baby knows when it is hungry, in pain, frightened, or happy. It needs no language to comprehend these states. But the baby does have to communicate right from the start. For this reason, all newborns communicate in the same language (crying and laughing, mostly). As newborns comprehend more and more of their environment through improved sensory equipment (eyes, ears, nose, fingers), they collect more and more data. At this point the various languages come into play, with different societies using different verbal symbols to express mental processes. The human brain still carries on the same nonlinguistic thought processes as before, however, because thinking in terms of symbols would take far too much time and “brain storage.”

The brain helps a human to transpose complex thoughts into a language, and vice versa, just as a computer translates programming languages into electronic impulses and vice versa. Imagine that a child is about to step in front of a speeding automobile. If the brain had to handle millions of data elements symbolically, we humans would spend all of our lives waiting for our brains to deliver the correct processed information, and that child would probably get killed before we could even begin to react. Rather, the brain scans all the data received by the sensory organs in real time and then sums it up into a single symbol for communication. A good audible (and hopefully loud) symbol in the above-mentioned case is “Stop!” You, seeing a child about to go out into heavy traffic, might shout that word and produce in the child’s brain the appropriate sequence of processes.

Not all languages involve the spoken word. Have you heard of sign language, whereby the arms and hands are used to communicate ideas? If you’ve done any amateur (or “ham”) radio communication, especially if you got your “ham” license back in the time when I got mine (the 1960s), you know about the Morse code as a set of communications symbols. In most instances, an entire communicating language of visual symbols is not as efficient for us humans as one composed of words and visual symbols combined. Using the symbol “stop” again, we can utter this word in many different ways. The word in itself means something, but the way we say it (our “tone of voice”) augments the meaning. We can’t do all that with the printed or displayed characters S, T, O, and P all by themselves in plain text.

We humans have arrived at some universal methods of modifying visual symbols. To many of us, the color red denotes something that demands immediate attention. Often, however, this color is used in conjunction with the visual symbol for a spoken word. Think of a “stop” sign, for example. It’s red, right? Or think of a “yield” sign. It’s yellow, representing something that demands attention, but in a less forceful way than the color red does.

Schematics don’t lend themselves to any form of oral (audible) symbology either. When you see the symbol for, say, a field-effect transistor (FET) in a schematic diagram, you don’t hear the paper or computer say, “Field-effect transistor, for heaven’s sake, not bipolar transistor!” You have to make sure that you read the symbol correctly. If you want to build the circuit and you mistakenly put a bipolar transistor where an FET should go—maybe because you didn’t look carefully enough at the schematic—you have no right to expect that the final device or system will work. Something might even burn out, so that when you recognize your error and replace the FET with a bipolar transistor, you’ll have to troubleshoot the whole circuit before you can use it!

Our senses along with our central processor, the brain, render us less than proficient at mentally conceiving all of the workings of electronic circuits by dealing with them directly. Therefore, we have to accept data a small step at a time, compiling it in hardcopy form (through symbology) and providing hardcopy readout. We can liken this method to the “connect-the-dots” drawings in children’s workbooks. Individually, the dots mean nothing, but once they are arranged in logical form and connected by lines, we get an overall picture. The dots’ relationships to each other and the order in which they are connected tell us everything that we need to know.

The remaining chapters in this book start with the symbols for individual electronic components, then move on to simple circuits, and finally show you a few rather complicated circuits. Schematic symbols and diagrams are designed for human beings, so human logic constitutes a prime factor in determining which symbols mean what. In that respect, the creation and reading of schematic diagrams resembles mathematics, and in particular, good old-fashioned plane geometry!