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How to Read Your First Autodesk EAGLE Schematic Like a Pro in 3 Easy Steps

Sam Sattel

my-first-schematic

How to Read Your First Autodesk EAGLE Schematic Like a Pro in 3 Easy Steps

The schematic forms the building block of every electrical circuit, and even if you aren’t designing one yourself, knowing how to read one is invaluable. And with some schematic reading knowledge in hand, you’ll be able to design, build and ultimately troubleshoot your way through your design logic before heading on to your PCB layout.

But how in the world do you go about reading a schematic if it’s your first time? There’s a ton of shapes that might be new to you, all those random lines connecting everything together. Fear not, we’ve got you covered with a surefire way to read your first schematic in Autodesk EAGLE like a pro, in only 3 steps.

Step 1 – Your Building Blocks

Let’s start by going over all the symbols you might find on a typical schematic. While we won’t be covering every possible symbol here, knowing the ones outlined below will make you knowledgeable enough to an average schematic. And for anything you don’t know, remember to refer to a part’s datasheet to find out. Here we go!

Resistors

Resistors are one of the most common building blocks you’ll find in an electrical circuit. Without these guys, those energy efficient LEDs that we’ve come to depend on would never work! The resistor symbol will look different based on if you’re using the US or international standards. For the US, you’ll find a zigzag line connected with two terminals. The international symbol is just a plain rectangle with the same terminal connectors.

Resistors

The resistor symbol in both US and International versions.

You’ll also want to be on the lookout for variable resistors and potentiometers. You can quickly identify these by looking for a diagonal arrow placed across the zigzag line for a variable resistor, and a horizontal arrow pointing towards the zigzag line for a potentiometer.

variable-resistors-and-potentiometers

With the help of some new arrows, we can easily identify a variable resistor and potentiometer.

Capacitors

Capacitors are those lovely little components that can hold some serious charge. You’ll find two kinds of capacitors, polarized and nonpolarized, each of which will have two terminals connected.

capacitors

Capacitors are easy enough to identify, with the polarized capacitor getting a curve for its cathode.

Inductors

Next up is the inductor. These guys are responsible for storing energy in the form of a magnetic field, all thanks to a simple coiled wire! Inductors are pretty easy to identify on a schematic, with the typical looping lines shown in US standards. International inductors are a bit more generic looking, and you’ll want to look for the solid rectangular box.

inductors

US inductors are easier to identify with their single looping coils.

Switches

These powerful little switches are responsible for throwing your circuit into action. When open, no electricity flows through, but in their closed state magic starts to happen. There are quite a few different forms of switches, but the simplest of the bunch is the single-pole/single-throw switch that has two terminals and a half-connected line. This half-connected line represents whether your switch is in an open or closed state.

spst

Here’s a switch with an open connection line and no electricity flowing through.

Power Sources

If you’re working with digital electronics, then you’ll likely only be dealing with DC voltage. But it’s also good to know what AC looks like should you ever stumble upon it on a schematic.

dc-and-ac-voltage-symbols

Your standard DC and AC voltage symbols for a schematic.

If your gadget is going to be powered by a battery, then your symbol will look entirely different. The battery symbol contains several lines, the top being the positive terminal and the bottom (and shorter) line being the negative terminal. And if your battery has more than one cell expect to see multiple lines.

one-cell-and-two-cell-battery

The common single-celled and two-celled battery symbol you’ll find on a schematic.

On more complex schematics you’ll likely see symbols to indicate node voltages. These nodes are used to connect devices to a single terminal and will be designated as either 5V, 3.3V, VCC (which is the positive supply voltage of a power pin), and Ground (GND). You’ll notice in the image below that all of the positive node voltages are shown with an up arrow or triangle, and every ground voltage is either an arrow, set of lines or triangle all pointing downwards.

power-gnd-symbols

All the variations you need to know for your positive and ground voltages.

Diodes

Diodes have many functions, the most common being the fabled LED. There’s also the photodiode, which can create energy from light by acting as a mini solar cell. You’ll find the standard diode symbol by looking for a triangle resting against a vertical line with two connecting terminals on each end.

diode-symbol

The standard diode symbol, not to be mistaken by a LED.

You can build off of this basic diode symbol to create LEDs and photodiodes by looking for a set of arrows. The arrows pointing away turn a diode into a LED, and arrows pointing toward the diode indicate a photodiode.

led-and-photodiode

With the addition of some directional arrows, we’ve created a LED and photodiode.

Transistors

You’ve probably heard about transistors large and small. They’re the granddaddy of the electrical symbols and are responsible for powering all of the modern complexity in your computer. And if these bad boys didn’t exist then you would never be reading this blog!

Transistors exist in two basic forms, as Bipolar Junction Transistors (BJTs) or Metal Oxide Field-Effect Transistors (MOSFETs).

You have two kinds of BJTs to look out for, both of which have three terminals for a collector, emitter, and base. You’ll notice in the image below that the emitter and collector pins are always in line with each other, with an arrow next to the emitter. The direction of this arrow determines whether the BJT is an NPN (Negative, Positive, Negative) or PNP (Positive, Negative, Positive) transistor.

npn-pnp

The two BJT transistors to be on the lookout for – NPN or PNP.

MOSFET transistors also have three terminals, but they’re called source, drain, and gate. You’ll find two different versions, an N-Channel and a P-Channel. The arrow found in the middle of each symbol below determines what kind of MOSFET transistor it is. An arrow pointing inward is an N-Channel MOSFET, and pointing out is a P-channel MOSFET.

n-channel-mosfet

N-Channel MOSFETs

p-channel-mosfet

P-Channel MOSFETs

Logic Gates

Logic gates power our digital, binary world of 1s and 0s. The standard logic gates consist of AND, OR, NOT, and XOR, each with its unique symbol.

logic-gates-and-or-xor-not

The building blocks of today’s binary logic gates.

 

If you go and add a circle to a logic gate, this creates an inverse effect of the gate’s value. For example, adding a circle to the AND, OR, or XNOR logic gates makes them NAND, NOR, and XNO.

nand-nor-xnor

The addition of a simple circle (on the right) turns these into NAND, NOR, and XNOR gates.

 

Integrated Circuits

Integrated Circuits, or ICs, are the behemoths of the electrical symbol world. These guys don’t have a symbol, since they do so darn much, but you can easily recognize them when you see a giant rectangular box with a bunch of labeled pins inside.

Each pin will be given a name that will tell what function it serves, along with a number to identify a pin connection. You’ll also notice that each IC has a unique name, and this is super important since each IC has the same shape.

integrated-circuits

Here’s three unique Integrated Circuits, all with a variety of pins and functionality.

 

There are a few ICs that have unique symbols, including the op-amp and voltage regulator. You’ll notice in the image below that the op amp has 5 terminals, whereas voltage regulars only require 3 terminals for input, output, and ground.

op-amp

The unique symbol for an Operational Amplifier.

 voltage-regulator

Voltage regulator.

Crystals and Resonators

You’ll find these precious crystals powering the most important parts of a microcontroller – the clock signal. These ladies have two terminals, whereas a resonator will have three terminals and two capacitors added into the mix.

crystal-resonator

Here’s the crystal, and her bigger sister, the resonator, with two capacitors added.

Headers and Connectors

If you’ve ever dug into the insides of your PC, then you’ll be right at home with headers and connectors. These come in a variety of symbols depending on their function, and you’ll find them responsible for providing power, distributing information or serving as connection points.

header-connector

A small sampling of the available connectors you’ll find on a schematic.

Transformers and Relays

You might have heard about those monolithic transformers that help to convert AC electricity into usable DC electricity in our homes. On a smaller scale, these devices are responsible for transferring power between two or more circuits, and you can identify the transformer by its dual coils.

transformer

Several variations of a transformer, each with its set of coils.

Relays will use the coil found in the transformer symbol and add an electromagnet and switch to power a circuit on or off.

relay

Here’s a relay, adding an open switch to the transformer coil.

Speakers, Buzzers, and Motors

Turn up the beats with some speakers, and buzzers! These simple devices look almost as they do in physical form and contain a positive and negative terminal.

speaker-buzzer

The speaker and buzzer are easy to identify with their almost lifelike shapes.

Motors make the world go round, and you can quickly identify these on a schematic by looking for a large M surrounded by a circle and a positive/negative terminal.

motor-symbol

When in doubt, look for the giant M to identify a motor symbol.

Fuses and PTCs

If you’ve ever had the power blow out in your house, then you know what fuses are all about! These guys help to limit any massive influx of current that can cause fatal damage to your sensitive electronic devices. The line connecting the two terminals of a fuse looks similar to a physical fuse, and this is where the connection point will separate when too much current flows through the fuse.

fuse

The simple fuse, brave protector of all our electronic devices.

A PTC, or positive temperature coefficient resistor, will vary in resistance based on its temperature. As the temperature increases so do the resistance of a PTC, and vice versa.

ptc

It’s getting hot in here! The resistance of this PTC increases with temperature.

That’s Enough to Make You Dangerous

That’s the end of step 1, and you now know enough schematic symbols to be dangerous. While we couldn’t cover every symbol known to man here, these basic symbols should get you 90% of the way when reading any schematic. If you ever need a helping hand remembering what a schematic symbol represents, try printing out this handy schematic symbol reference chart.

Step 2 – Connecting It Up

Now that you have a good understanding of all the basic building blocks of your schematic symbols, it’s time to understand how everything gets connected. There are two main elements to this step, connection points, and symbol identification.

Connect the Nets

All of those lines that you see on a schematic connecting your symbols are called nets. These nets will tell you exactly how components need to be wired together on your breadboard or PCB layout. Here’s a simple circuit below that shows the nets in a different color from the schematic symbols:

led-circuit

Nets come together to connect all of your schematic symbols.

Simple enough, right? Things can get a little complicated though when you start to build some complexity into your schematic. If you happen to see a wire splitting into multiple directions, it can mean one of two things:

It’s a node

It there’s a node, or solid circle, in the middle of a pair of intersecting wires then this is a junction. This means that the wires are electrically connected, and current will be heading in both directions.

junction-point

Be on the lookout for a node to identify net connection points.

It’s an overlap

If you don’t’ see any node in the middle of two intersecting wires, then you’ve got two separate wires that don’t share any electrical connection. And while we recommend avoiding this kind of overlap to avoid confusion, sometimes it just can’t be helped.

not-connected-circuit

Here are two wires that intersect but don’t share any electrical connections.

You might also run across some nets on your schematic, specifically near integrated circuits, that end in a label without actually being connected to anything. These net names are used to reduce wiring clutter on your schematic, but it’s assumed that they are connected by their label.

ports

The nets on this integrated circuit end abruptly, but are still connected.

Give It a Name

Now that you understand how all of those symbols are connected with nets let’s talk about all of the strange names that you’ve seen next to each symbol on a schematic. There are two parts to this, the name and value.

Names

Every symbol on a schematic is given a name and number. This helps to provide a unique identifier for each symbol, especially when you have multiple components of the same type being used. Symbol names will typically be a combination of a letter and a number, with the letter identifying the type of part, and the number being the unique variation of that symbol.

For example, a resistor will be labeled with an R. IF you have multiple resistors to place, then they will follow a logical numbering sequence of R1, R2, R3, etc… Some other component letters to be aware of include:

  • C – Capacitors
  • D – Diodes
  • L – Inductors
  • Q – Transistors
  • S – Switches
  • U – Integrated Circuits
  • Y – Crystals and Oscillators

Values

Values listed next to a symbol will help to identify how many ohms, farads or henries a particular component has. Other symbols might use the value field to specify the name of an integrated circuit or the oscillating frequency of a crystal. Regardless of which value is listed, know that it’s the primary value you need to know for a part and is displayed for a reason.

Here’s an example putting both the Name and Value concept together. As you can see in this circuit, we’ve got a resistor, labeled R1, which means it’s the first resistor in our schematic. And 1k Ohms is its value.

res-value

Every symbol on your schematic needs a name and value, just like this resistor.

Step 3 – Putting It All Together

We’re now at the final step of our schematic reading journey, the actual reading part! You can think of reading a schematic kind of like reading a book. You’ll always read from left to right, top to bottom. And every circuit will flow from input to output (AKA power to ground). In the example below, we have a super simple circuit for a LED flashlight. While your schematic will probably have way more stuff, this will give you a decent idea of how things flow together.

led-flashlight

A very simple circuit for a LED flashlight, ready for reading!

By following this circuit like a book, we start the left at our power source, which happens to be a battery. Moving with the flow of power, we then hit an open switch. When the switch closes, electricity will keep flowing through to hit a resistor which then will make the LED light up without burning out!

You Are Now Schematic Literate

So there you have it, everything you need to know to be able to read your first schematic in Autodesk EAGLE like a pro! If you want to see all of this knowledge in motion be sure to check out this excellent video by Ben Heck on the Principles of Schematics. One final thing to keep in mind, as you begin to write your schematics remember that another engineer will likely see it down the line, whether that’s for a design review or to build another project. Do yourself and your fellow engineer a favor by making sure that all of your symbols are properly labeled, and even break things down into logical functional blocks to make it all easy to read.

Ready to get started on making your schematic? Try Autodesk EAGLE for free today!

 

Source: Sparkfun

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