How to read circuit diagrams

Alan Winstanley, of Everyday Practical Electronics Magazine explains everything you need to know about understanding circuit diagrams properly.

Whether you are an electronics hobbyist or a budding auto-electrician, if you’re struggling to get to grips with circuit diagrams then this simple tutorial is for you!

Basic principles

If you’ve come across a circuit diagram for the first time – maybe in a hobby electronics magazine like Everyday Practical Electronics or even a Haynes car repair manual – then you can be forgiven for being a bit confused. A diagram full of spaghetti-like lines and weird symbols – how on earth do you make sense of it all?

In this special article I’ll explain how to interpret even complex circuit diagrams properly and generally find your way around them with confidence. Most seasoned electronics hobbyists and technicians can read them like they were written in plain English, and actually it doesn’t take much practice to understand them, as you’ll soon discover.

When our elder sister magazine Practical Electronics was launched back in 1964, each and every circuit diagram and assembly drawing was expertly and beautifully drawn entirely by hand, using fantastically skilled artists and draughtsmen. These days, computer software is used to draft circuit diagrams on-screen (schematic capture), offering us the bonus of being able to re-arrange parts with a mouse-click to obtain the best-looking diagram layout. The design data can then be fed into a printed circuit board design package and a whole PCB can be designed and manufactured from that.

Whether hand-drawn or produced on a CAD package, there are some basic principles that are common to all circuit diagrams. Good circuit diagram design helps ensure that anyone can pick up a diagram and understand clearly what is going on. It’s the responsibility of circuit designers to ensure that their diagrams are legible and easily understood, to explain graphically the make-up of a circuit and to help those who need to work with it afterwards, either in manufacture or when repairing an item in the field.

Mapping it out

If you’re new to the subject of reading circuit diagrams (also called circuit schematics), then one way to start is to think of them as road maps. The “towns” and “villages” in the map are the electronic components themselves, and the lines representing “highways” or “roads” show how they’re connected to each other.

Sometimes there will be “intersections” or “junctions” where the roads meet each other and are joined together. This is where electronic components are physically connected with each other.

Other times, we’ll definitely not want them to meet each other! For example a road bridge passing over a road underneath; in electronics terms, these are times when electrical wires or conductors are fully insulated from each other and our circuit diagrams need to show that very clearly. Just imagine the potential problems if we can’t figure out if something is supposed to be connected to something else!

On maps there’ll be narrow single-lane “roads” carrying some traffic – just like a wire carries an electrical current or audio signal, but you’ll sometimes also see huge multi-lane highways carrying loads of traffic all side by side!  What’s more, there might be high-density roads carrying very heavy traffic.

Apart from visualising those little villages on our imaginary roadmap, you’ll see large areas representing conurbations and built-up areas like towns and cities. Roads and highways are still represented as solid lines connecting these places, but because the cities have extremely complicated road networks within them (think of London or Paris!), the map doesn’t show each and every twist and turn of the city roads. They’re simplified as big areas coloured on a map. You’d have to buy an A-Z guide to see the internal road network of a city itself.

A circuit diagram can work the same way, by showing boxes that represent something (e.g. an integrated circuit) that’s a lot more complicated internally, or something that could even justify having its own circuit diagram.

One major difference between our road map analogy and a circuit diagram is that a map is geographical: it has to show the location of roads and towns in correct relation to each other. (One exception is the world-famous map of the London Underground, a design masterpiece that shows which tube stations are on which line. But not geographically.)

A circuit diagram has no geographical handicaps: as long as the solid lines go to the right places, it doesn’t matter where the components are shown on the circuit diagram relative to each other. Good circuit diagram design, though, mean that parts are grouped together reasonably closely on the diagram just as they would be in the circuit board itself, so that parts aren’t scattered randomly around in the drawing.

In a nutshell then, when you see a circuit diagram, simply think of it as a road map stripped of all unnecessary detail.

At a junction

If we’re going to understand how to read circuit diagrams, then the basics start with the wires – the conductors that carry electrical current from one component to another. Without them you don’t have a circuit!

Juntion of two conductorsIn a circuit diagram, solid lines indicate wires. If several wires join each other we always show this with a blob.

No junction where lines (wires) cross!No jucntion, as shown by the hoopIf lines on a diagram cross each other but you don’t see a blob then the crossing wires are not joined to each other. Sometimes you’ll see a ‘hoop’ to emphasise that wires aren’t joined together. But that’s a bit old-fashioned these days.

The most basic electronic components (resistors, capacitors, diodes, transistors etc.) are the building block “houses” of our circuit, often called discrete components, as opposed to integrated circuits which are silicon chip “cities” on our map. Components are the destinations in our roadmap, interconnected by wires (“roads”).

Each component has a unique symbol, and it’s important that you learn to recognise what they are. After a little experience you’ll soon be reading diagrams with ease. Sometimes the symbol also attempts to explain how the component works, such as the one-way arrowhead of a diode (which only conducts one way, but not the other), or a capacitor, which is physically divided by an internal gap (filled with a dielectric). Components are labelled with unique serial numbers, e.g. resistors start with R1, R2 etc, capacitors are labelled C1, C2 in the circuit diagrams, and so on. Either their values will also be shown in the drawing or there will be a separate components list.

No need to show blobs joining componentsUsually, a solid line on a circuit diagram will connect directly to one component such as a resistor (shown left), so there’s no need to show a blob where it joins onto the resistor, otherwise the diagram becomes messy and confusing to look at.

Wrong way to show wires joining each other on a circuit diagramPreferred way of showing a junctionIt’s best practice never to show a “crossroads” of wires with a blob, but to offset them as shown. Then it’s crystal clear what the diagram represents, and you’ll be left in no doubt that all four wires are joined together.

Remember, a circuit diagram has to be drafted so that someone who’s never seen it before – a repair technician for example – can easily understand what the circuit represents without making mistakes.

In summary:

  • Joints or junctions where wires or components are electrically connected to each other are represented by a “blob”.
  • Lines that cross each other but do not have a blob, are not connected to each other.
  • Avoid “crossroads blobs” to improve confidence when reading the diagram.
  • There’s no need to show blobs when connecting to individual components.
  • All components have a unique ID, such as R1, R2, etc. and values may also be shown (e.g. R1 100 ohms).

Riding on (power) rails

All circuitry needs a source of power, whether derived from the mains electricity, or from d.c. batteries.

A battery has two terminals marked + and -.  A car battery usually has +12V and -12V markings. (It’s pedantic but technically correct to say that the use of +12V and -12V is misleading, because it implies that mathematically there’s a potential difference (voltage) of 24V between the terminals: its markings should really is +12V and 0V. But ordinary consumers just know them as the positive and negative terminals.)

The idea of 'negative earth' is like a 'ground' connectionA car is the perfect example to illustrate my next point: the principle of ‘grounding’.  Most cars are made of steel and have a 12V ‘negative earth’ or ‘negative chassis’. What this means is that the metal body of the car acts as a huge continuous ‘wire’ connecting directly to the negative terminal of the battery.

Car makers do this to save miles of wiring: a headlight just needs a single +12V wire to go to the bulb from the headlight switch, and the bulb’s other ‘wire’ is actually the entire car body. Current then flows through the steel body and through an earth strap straight back to the car battery, thus forming a circuit. The same is true of most of the car’s electrics, from the smallest bulb to the starter motor: there is a ‘live side’ which is at +12V and then there’s a ‘return’ which goes to the metal chassis and back to the battery negative.

Earth and Ground symbolsThis feature of having a “chassis” or “ground” to act as a giant “common” wire is used a lot in circuit diagrams, because it saves designers having to draw lots of lines to join up the ‘negative side’ of a circuit.

You’ll frequently see a special symbol to indicate a “ground connection”. Somewhere in the circuit diagram, a battery or power source often has one terminal (usually 0V or the ‘negative’) connected to “ground”. Elsewhere in the circuit, components will also be shown connected to ground.

Here are two points to remember:

  • Ground symbols are used to keep circuit diagrams clear and simple: everything having a ground symbol is connected together, it’s just that the wires aren’t drawn in the circuit diagram itself. Consider them as invisible “common” connections through a car body.
  • A different symbol is seen for “earth” connections. These are literally connections to the soil, e.g. for safety reasons. But you also see this symbol being used to represent “chassis” or ground connections (like a car body). In the USA they tend not to say “earth” at all but “ground” instead. However in Britain, an “earth” connection usually means precisely that.

The wires that carry the main source of power around the circuit are usually called supply rails. Remember in electronics, we can’t afford to use incorrect car battery terminology – we’ll properly say e.g.  +12V rail or 0V rail or ground rail. In electronics, a negative rail is indeed less than 0V! This sounds impossible: how can you have something less than zero?

Split supplies, showing +12V, 0V and -12VIt’s a simple matter of relativity. For example, in a dual rail supply circuit you might see three supply rails –  let’s say + 12V, 0V and -12V  (shown), We won’t go into circuit design here but the +12V rail could equally be labelled as +24V,  the mid-way 0V rail could be labelled as +12V and the -12V rail labelled as 0V: the voltage differences in relation to each other would still be the same. So don’t be put off by the concept of ‘negative voltages’, it’s just the way we denote all voltage levels in relation to a 0V rail.

It is however preferable to show a “0V common rail” in a circuit diagram and label the other voltage rails with respect to that. Often the 0V side is grounded as depicted by the use of the grounding symbol; everything with a ground symbol then connects together.

Sometimes you might find that the thickness of the lines in a circuit diagram varies. This is a bit unofficial but it’s used to show how “heavy duty” the wire is, for example, a connection to a starter motor or a mains heater. All of these ideas help the technician to visualise at a glance what’s going on when they study the circuit diagram.

  • Supply “rails” are the voltage supplies used to power the circuit. They will be clearly marked with their voltage.
  • Take care to understand the idea of relative voltages – for instance a +12V supply rail will be 12 Volts positive with respect to (w.r.t.) the 0V rail. Negative voltage rails will be negative w.r.t. the 0V rail.
  • Thick lines in circuit diagrams can represent heavy loads, to help highlight them in the circuit schematic.
  • Now you can explain to your car-mechanic friends why all along they’ve been wrong to talk of +12V and -12V of a car battery!

We’ve now seen how circuit diagrams describe how components are joined into a circuit, how “blobs” indicate junctions in wiring, and we’ve explained how power rails and ground symbols show how supply voltages are distributed around the circuit. Let’s move on from our simple car battery idea and look at electronic circuit diagrams proper.

Buses and Interwiring

Apart from simple discrete components joined together by wires (or the copper tracks of a printed circuit board), you’ll see integrated circuits (i.c.s) or chips used at the heart of circuitry. Integrated circuits can have anything from four pins to the many hundreds seen on central processing units.

IC’s can be considered to be conurbation “cities” in our road map analogy. They can contain millions of transistors which are obviously impossible to represent in a circuit schematic (unless you want a sheet of paper larger than several soccer pitches). Therefore, only the “city limits” are shown in a circuit diagram. An integrated circuit is shown by a simple outline box with each pin number brought out separately. In simpler chips, the pins might be labelled to indicate their function, if there’s room on the diagram.

The same technique can be used in diagrams for dealing with complex sub-systems or even connecting different circuit boards together. Each sub-system or circuit board can be represented as a simple black box with terminals (inputs and outputs) connected by interconnecting wires shown as lines in circuit diagrams. Below is a block diagram of our MicroLab single board computer.

Block diagram of MicroLab, showing various buses

Imagine that you have a hugely complex circuit containing logic or CPUs or digital displays. It becomes messy if not impossible to have a circuit diagram showing thousands of wires joining various parts together. A schematic shorthand we use is to draw a “bus” – a line representing a group of connections, such as an “address bus” or “memory bus”.

  • The idea of “buses” in schematics saves having to draw hundreds or more of lines side by side on a circuit diagram.
  • The inter-connections of sub-systems or even entire circuit boards can also be depicted using black boxes linked with wires and buses.

The skilful use of lines and buses in circuit diagrams shows the reader how everything is connected together, leaving no room for doubt. Now that you know the basics, let’s see how some unusual components are represented in circuit diagrams.

| On to page 2 | TOP |