Fundamentals

# Circuit Diagrams

Daniel Koch

Issue 7, January 2018

In this month’s Fundamentals, we’re going back to basics – learning how to read… circuit diagrams, that is!

In the English language, symbols are used to represent a certain piece of information. These are, of course, letters! Each letter can be strung together with other letters to make a word, and words strung together to make a sentence. There are rules that give a consistent order to what letters combine to make a word, and what words combine in which order to make a sentence. It is these rules, and the consistency they establish, which allow communication to take place.

I detail this because most of us are familiar with the principles of reading and writing. However, some may not be aware that the same concept applies to circuit diagrams. Circuit diagrams are the graphical representations used to show how all the components on a circuit are connected to each other. And much like the English language, these drawings also follow certain rules, and once understood, these rules guide our interpretation of the information provided.

The first rule is the consistency of symbols. Like letters in an alphabet, symbols in a circuit diagram are standardised so that they always mean the same thing. Learning these is unfortunately much harder than learning the 26 letters of the English alphabet because there are many more of them, and variations of each; a bit like the accents applied to some letters on languages other than English to change the way a letter is used. Coffee at the café anyone? Items such as integrated circuits are often represented differently depending on their unique properties. Thankfully, like the accent on the ‘e’ in ‘café’, the overall appearance of the symbol is recognisable, so the details can then be examined.

There is further analogy between circuit diagrams and languages. Like English, symbols in circuit diagrams are different around the world. The American symbol for a resistor, for example, looks like a zigzag line, while the symbol in most other countries is a rectangle [1]. However, they are used in the circuit the same way, just like how a word is used the same way in a sentence even if it is spelled differently – take ‘colour’ and ‘color’ as an example. Additionally, circuit diagrams follow rules of directionality. In English, we read left to right, then drop down to the left of the next line, and read left to right again. We continue down the page. Circuit diagrams are more diverse, but all are still read according to one rule – conventional current flows from positive to negative. Therefore, once the power supply is identified, connections can be followed through each component, one after another, to ground. Just as we would not look at a whole paragraph of text at once, we don’t need to try and absorb the whole diagram at once; instead, we break it down to one current path at a time.

A personal favourite technique, one I still use with more complex circuits and that I demonstrate to students in the primary school classroom, is to use a highlighter or marker pen to trace the lines as I read the diagram. Over the top of the paper, place a clear acetate sheet – such as those found on some cheap display folders or used as the cover in binding machine – and you can then erase and repeat while preserving your original diagram.

This simple transistor circuit [2], which comes straight out of Issue 6. Find the line marked either Vcc or 5V DC (or 12V, or whatever the supply voltage is). Starting here, trace forward until you encounter a junction. Move downward from the line until you encounter the transistor Q1. You can trace until you get to the flat line that joins emitter, collector, and base. At this point, the line you have drawn is inviting you to consider what the component is doing. In the case of this transistor, you can see that, if the base had its input signal (which you may assume or draw in, perhaps in another colour), then current would flow to the base of Q2 [3]. This shows you, as you pass through Q2, that there is a current path to it from the supply rail. You may draw this in, either in the colour you are using, or another. At this point, the current path continues to Q3, and then to ground.

When most of us were at school, we were taught that there was one right way to do things, and that everything else was wrong. In modern teaching, research supports the idea that there are many correct ways, and that each child must experience them all and choose which one works for them [4]. I have simply chosen to trace the current paths in the most direct line from Vcc to ground [4], as opposed to following the most detailed route [2]. The colours should help show what I mean far better than words can.

Now I would love to say that all circuit diagrams can be read from left to right, top to bottom. However, this is not always the case. As there are many languages around the world with their own conventions, in the electronics world, circuit diagrams can also be drawn in different ways. In understanding them, my advice is to use some bushcraft! For example, when reading a map, the first thing we do is find north, and orient the map to face that way. In a circuit diagram, the first step is to find the supply rail and the ground connection. Once this is done, the overall orientation of the diagram can be gleaned. After this, the major components can be identified (and highlighted if that helps). Now we can begin tracing current paths from positive to negative.

Finding the ground in some circuits is harder than in others. This is often because some diagrams just use the earth symbol at various points around the circuit to show the connection, and don’t actually feature a 0V rail [5].

In this example of a 555 in astable mode, (don’t worry if you don’t know what this means, it’s not important to your understanding of what connections go where), the connections to supply and ground don’t go to a continuous line at the top and bottom. Instead, the connections to +5V end at a T-shaped line marked Vcc, while the connections to ground have three progressively smaller lines.

Following the current path in these diagrams is the same in theory as in diagrams with rails drawn [2], but in practice you can’t always physically trace it all the way. My advice here is to colour-code the connections to ground and Vcc (often indicated at the side of the diagram where the voltage of Vcc is established – i.e., the top left of the 555 circuit).

After a while, the coloured markers will become less necessary. However, they have one last place. Highlighting each connection on a circuit diagram as you physically make the connection on a prototyping board or circuit board can reduce headaches later by avoiding missed or incorrect connections. I still do this unless I’m rushing.

Here are my suggestions for reading circuit diagrams:

• Highlight the entire voltage supply rail, in a unique colour.
• Highlight the whole ground rail, again in a unique colour.
• Identify the major components.
• Begin highlighting current paths, using either method. You may even come up with your own, in which case just make sure it is repeatable and works in all situations.

Note that some components will only allow current to flow one way. Consider at each of these, where the current will go as your line passes through. In non-polarised components such as resistors, current flow will be determined by the other components, or simply by the way current flows from positive to negative. This is why drawing coloured lines on the diagram can be helpful.

And there you have it! You are now on your way to reading even the most complex circuit diagrams.

We would love to show you a comprehensive chart of all of the components, their symbols, and variations; however, we’ll save this for a future inclusion, as there are a great many to cover – so watch this space! Of course, you can also refer to our series "The Classroom", where we explore basic components and include their circuit symbols and variations.

So that’s all for this month. We would love to hear from you, especially if you develop your own repeatable way of reading circuit diagrams; after all, this is a community that is built on a foundation of sharing!