Schematics – Reading and Writing in Electronics.

Published by Gaz on

Stock image of board found on pexels

Schematics - How to Read and Write in Wires and Electrons

Music has staves and notes. English and many other languages use a selection of characters and accents to convey a word or a meaning or an idea.

But how can we convey a circuit? What system can we use to say, if you connect a battery to this thing it will do what we want it too, or if we change this part to a higher value it will emit a lot less smoke and not cause a house fire?

This is an introduction to schematics. How to read them. The symbols we use when we write them. And also, how to translate from American style schematics to the rest of the symbols of the world.

Making a connection

The best circuit diagrams are the ones that are easiest to read. Saying that; this doesn’t mean that components are in the order or placement that they will end up on the board. It just means that we can follow, somewhat logically from one point to another. 

When we’re actually designing our PCB it may be a lot easier to have, for instance, the logic portion of our circuit all grouped around one side of the board, but in a schematic, we might be able to read it better if it’s in the middle. Or even have it pushed closer to the individual parts it’s controlling.

Getting Our Wires Crossed

The biggest concern about schematics lies in the fact we are faced with two competing standards.

HackerSpace will always try to use the European or IEC standards. However, it is good to know that these alternatives are in use and how to spot them.

Let’s discuss our first schematic. A simple battery to LED schematic.

Simple Schematic of an LED and Battery Circuit

Pro Tip

Schematics are typically read from the top right to the bottom left, like English. However, it’s not universal.

We also tend to put positive voltage potentials up and ground or negative potentials down for ease of reading.

LED & Switch

This schematic has 4 components connected together.


  • A Battery; indicated by the long thin and short fat alternating lines.
  • A Switch; indicated by the two terminals with a disconnected bar across them.
  • An LED; indicated by a diode, with arrows pointing away, because.. you know. Light.
  • And, A resistor; Indicated by an empty block.

As we see from the above diagram there is a battery on the left. This is marked as a 6V but as we’ll learn in our journey through electronics, a wide range of good enough and close enough will get the job done.

This can be anything over the forward voltage of the LED as long as we soak up the excess current with the resistor (R1).

We’re going to aim for 9 mA here.

6v (Battery) - 1.8v (LED Vf) / 470Ω (resistor) = 0.009A

0.009A / 1000 = 9mA

Plug your own values in to the above values and you can substitute what you see with what you have!

5mm LED’s have can easily handle 20mA so even if we swap up to a 9 volt battery, we will still be within the safety margin for both the LED and the resistor.

When in doubt, you can always use ohm’s law or our ohm’s law calculator by clicking here.

100 Ways to Skin an Electron

Getting Wired

Wire junction example as used in schematics

Wire Junction

When we want to connect two or more points in a circuit, we connect them by drawing a circle or dot at the intersection.

Wires That Don't Connect

Conversely, when there is a need for two wires to cross but not connect we do not draw the dot.

Crossed wires example as used in schematics

In older schematics, it’s not uncommon to see this written as, touching is connected, with or without the dot and one of the non connecting wires jump  over the other.

This is also common when hand drawing schematics to make it very clear that these lines do not interact at all.

Power Sources & Sinks

VCC flag as used in schematics

VCC

VCC, in its own right, stands for Voltage, Collector Supply. Other Variants such as VDD, VSS, & VEE all stand for typical rail voltages and can be either positive or negative with reference to ground.

It is often used as shorthand just for a particular voltage range. So if we have a schematic for a particular circuit that can run between 5v and 12v we can use VCC as our “input voltage”.

+5V

+5V, and other positive voltages are written simply as their value over the platform and connected in wards.

Positive potentials are typically placed near the top of our schematics

+ 5 Volt flag used in schematics
-5 Volt Flag used in schematics

-5V

-5V, and other negative voltages, in contrast, fall more often on the bottom of the schematic and below the Ground connection on the circuit.

Generic Input / Output Flag

When we want to connect a signal from the output of another, for instance, a micro controller pin or, to the input of another source, such as… oh I don’t know, a micro controller pin, we use a flag similar to this one.

Alternatively we can use a flag with the name of the connection inside it.

Generic input and output as used in schematics
Earth ground flag as used in schematics

Ground with Reference to Earth

This flag is used when we need to connect a wire or component to earth with respect to earth.

In most countries this would be the earth pin of your wall plug.

Ground for Signals

This flag is used when we wish to connect a wire or component to earth with reference to the signal ground, if it happens to be different.

Signal ground as used in schematics
Chassis ground flag as used in schematics

Grounded to a Chassis

This flag is used to indicate when a component or path needs to be earthed with respect to the chassis.

Typically, chassis is also references to earth also.

Generic Ground

Sometimes we will find an ambiguous ground symbol like this. It’s left to the engineer to decide which grounding is most suitable.

Either that or it’s not wildly important.

generic ground flag as used in schematics

Resistors

Resistor symbol as used in schematics

Resistor

Our little friend, the resistor.

Drawn as a nice square block, neat and clean. Don’t not trust the false zig-zag resistor God.

Variable Resistor

If we have a resistor, and we take an output from a point other than the end then we probably have a resistor we can change the value of.

So, a variable resistor. (potentiometer or fader in some circles).

Potentiometer symbol as used in schematics

Capacitors

Non polarised capacitor symbol as used in schematics

Non Polarised Capacitor

Capacitors are two plates close together that have a small electrolyte between them so what better way to draw them as two plates with a space between them?

Polarised Capacitor

That is, of course, unless it’s a polarised capacitor like an electrolytic capacitor. 

We denote negative by thickly colouring it and for added clarity we can draw a + at the positive voltage potential.

Americans would typically use a bent or curved surface to denote negative.

Polarised capacitor symbol as used in schematics

Diodes

Diode symbol as used in schematics

Diode

Your typical diode. current goes through the triangle to the point and is blocked by the wall.

Pretty analogous for its function actually.

Light Emitting Diode (LED)

The LED symbol looks almost identical to the diode and for good measure. 

The only difference, really is that the LED emits exactly two straight lines of light*

*NOTE: Your photons may appear in more or less directions than stated in the above joke.

For more information check out our post on Understanding LEDs

LED symbol as used in schematics

Zener Diode

Zener diodes (remember Z for zener) are shown by altering the final bar of the diode symbol.

Zener diodes have a breakdown voltage that will allow electricity to pass in reverse. This can be useful for clamping a voltage.

Got a 16v input but your IC only takes a max of 12v? This is where our friend the zener comes in!

Schottky Diode

Schottky diodes (remember S for schottky) are shown by altering the final bar similar to a zener but in a curled almost reverse S shape.

Schottky diode symbol as used in schematics

Switches & Buttons

Switch symbol as used in schematics

Generic Switch

A switch is a simple device that either connects or disconnect a path.

The symbol makes sense when you look at it.

Push Button

Similar to a switch, a button makes to contact by connecting two parts of a circuit. There are two types, Shown here is a non latching push button meaning it will only be active or on, as long as its depressed.

Button symbol as used in schematics

Tubes and Transistors

Generic valve symbol as used in schematics - in this case one side of a 12ax7

Valves / Vacuum Tube

Each valve, or for our American friends, vacuum tube, will have it’s own symbol but it typical looks similar to the one on the left. It shows the connection between the grid(2), cathode(3) and anode(1).

NPN BJT

NPN Type Bipolar Junction Transistors (BJT’s) are drawn with the arrow facing away from the base.

NPN BJT transistor symbol as used in schematics
PNP BJT transistor symbol as used in schematics

PNP BJT

PNP Type Bipolar Junction Transistors (BJT’s) are drawn with the arrow facing towards the base.

N-Channel MOSFET

Similar to the BJT, Metal Oxide Semiconductor Field Effect Transistors are shown having an arrow.

However, the N-Channel MOSFET will have one that points in.

N-Channel MOSFET symbol as used in schematics
P-Channel MOSFET symbol as used in schematics

P-Channel MOSFET

Similar to the BJT, Metal Oxide Semiconductor Field Effect Transistors are shown having an arrow.

However, the P-Channel MOSFET will have one that points out.

Gates & Integrated Circuits

Generic IC

IC’s, or integrated Circuits will typically be drawn as a square box with a line out for each pin.

Note, there is usually pin numbering on the IC symbol as more often than not, the IC symbol will not have it’s pins in order to make reading the schematic simpler.

NE555P symbol as used in schematics
The Schematic Symbol for a NAND Gate

Logic Gates

Each Logic gate has it’s own variation of inputs and output we discuss them more in depth in our posts;

Computational Logic – A Beginners Guide to Logic Gates – Part 1

Computational Logic – Building it from the Ground Up – Part 2

Best Practice

There are few hard and fast rules. The most important thing is that you and whoever else needs too can read your schematics.

That being said, we typically don’t want a rats nest of lines going in seemingly every direction, so, here’s a list of our top tips for clean schematics.

  • Highest Potential at the Top
    • We always try to draw our schematics with the highest voltages near the top of the diagram and ground or negative voltages on the bottom.
  • Straight lines
    • Always prioritise North / South & East / West lines.
    • You can use angled lines too but these should be at 45˚ when possible and always should be secondary.
    • Always do your best to avoid loops and curved lines.
  • Give yourself more space.
    • You need to give yourself extra room when drawing schematics. Maybe you don’t think so, those components are so close together. Just leave the space. Sometimes you’ll want to add values or notes in and you will need it. Even if it’s just component numbers.

This should cover the most common symbols you’ll come across in your experiments but every now and then new symbols are created, modified and bastardised.

This might be for ease of drawing, especially in older ascii schematics or handwritten diagrams.

Find a symbol you don’t recognise? Drop it in the comments below and lets work out what it is!


Gaz

Electronics and Audio nerd.

0 Comments

Leave a Reply

Your email address will not be published. Required fields are marked *