FIXLEDTV: January 2014

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Schematics
are our map to designing, building, and troubleshooting circuits.
Understanding how to read and follow schematics is an important skill
for any electronics engineer.

This tutorial should turn you into a fully literate schematic reader! We’ll go over all of the fundamental schematic symbols:

Then we’ll talk about how those symbols are connected on schematics
to create a model of a circuit. We’ll also go over a few tips and tricks
to watch out for.

Schematic Symbols (Part 1)

Are you ready for a barrage of circuit components? Here are some
of the standardized, basic schematic symbols for various components.

Resistors

The most fundamental of circuit components and symbols! Resistors on a schematic are usually represented by a few zig-zag lines, with two terminals extending outward. Schematics using international symbols may instead use a featureless rectangle, instead of the squiggles.

Potentiometers and Variable Resistors

Variable resistors and potentiometers each augment the standard
resistor symbol with an arrow. The variable resistor remains a
two-terminal device, so the arrow is just laid diagonally across the
middle. A potentiometer is a three-terminal device, so the arrow becomes
the third terminal (the wiper).

Capacitors

There are two commonly used capacitor symbols. One symbol represents a polarized
(usually electrolytic or tantalum) capacitor, and the other is for
non-polarized caps. In each case there are two terminals, running
perpendicularly into plates.

The symbol with one curved plate indicates that the capacitor is
polarized. The curved plate represents the cathode of the capacitor,
which should be at a lower voltage than the positive, anode pin. A plus
sign might also be added to the positive pin of the polarized capacitor
symbol.

Inductors

Inductors are usually represented by either a series of curved bumps,
or loopy coils. International symbols may just define an inductor as a
filled-in rectangle.

Switches

Switches
exist in many different forms. The most basic switch, a
single-pole/single-throw (SPST), is two terminals with a half-connected
line representing the actuator (the part that connects the terminals
together).

Switches with more than one throw, like the SPDT and SP3T below, add more landing spots for the the actuator.

Switches with multiple poles, usually have multiple, alike switches with a dotted line intersecting the middle actuator.

Power Sources

Just as there are many options out there for powering your project, there are a wide variety of power source circuit symbols to help specify the power source.

DC or AC Voltage Sources

Most of the time when working with electronics, you’ll be using
constant voltage sources. We can use either of these two symbols to
define whether the source is supplying direct current (DC) or
alternating current (AC):

Batteries

Batteries, whether they’re those cylindrical, alkaline AA’s or rechargeable lithium-polymers, usually look like a pair of disproportionate, parallel lines:

More pairs of lines usually indicates more series cells in the
battery. Also, the longer line is usually used to represent the positive
terminal, while the shorter line connects to the negative terminal.

Voltage Nodes

Sometimes – on really busy schematics especially – you can assign
special symbols to node voltages. You can connect devices to these one-terminal
symbols, and it’ll be tied directly to 5V, 3.3V, VCC, or GND (ground).
Positive voltage nodes are usually indicated by an arrow pointing up,
while ground nodes usually involve one to three flat lines (or sometimes
a down-pointing arrow or triangle).

Schematic Symbols (Part 2)

Diodes

Basic diodes are usually represented with a triangle pressed up against a line. Diodes are also polarized,
so each of the two terminals require distinguishing identifiers. The
positive, anode is the terminal running into the flat edge of the
triangle. The negative, cathode extends out of the line in the symbol
(think of it as a - sign).

There are a all sorts of different types of diodes, each of which has a special riff on the standard diode symbol. Light-emitting diodes (LEDs) augment the diode symbol with a couple lines pointing away. Photodiodes, which generate energy from light (basically, tiny solar cells), flip the arrows around and point them toward the diode.

Other special types of diodes, like Schottky’s or zeners, have their
own symbols, with slight variations on the bar part of the symbol.

Transistors

Transistors, whether they’re BJTs or MOSFETs, can exist in two
configurations: positively doped, or negatively doped. So for each of
these types of transistor, there are at least two ways to draw it.

Bipolar Junction Transistors (BJTs)

BJTs are three-terminal devices; they have a collector (C), emitter
(E), and a base (B). There are two types of BJTs – NPNs and PNPs – and
each has it’s own unique symbol.

The collector (C) and emitter (E) pins are both in-line with each
other, but the emitter should always have an arrow on it. If the arrow
is pointing inward, it’s a PNP, and, if the arrow is pointing outward,
it’s an NPN. A mnemonic for remembering which is which is “NPN: not pointing in.”

Metal Oxide Field-Effect Transistors (MOSFETs)

Like BJTs, MOSFETs have three terminals, but this time they’re named
source (S), drain (D), and gate (G). And again, there are two different
versions of the symbol, depending on whether you’ve got an n-channel or
p-channel MOSFET. There are a number of commonly used symbols for each
of the MOSFET types:

The arrow in the middle of the symbol (called the bulk) defines
whether the MOSFET is n-channel or p-channel. If the arrow is pointing
in means it’s a n-channel MOSFET, and if it’s pointing out it’s a
p-channel. Remember: “n is in” (kind of the opposite of the NPN
mnemonic).

Digital Logic Gates

Our standard logic functions – AND, OR, NOT, and XOR – all have unique schematic symbols:

Adding a bubble to the output negates the function, creating NANDs, NORs, and XNORs:

They may have more than two inputs, but the shapes should remain the
same (well, maybe a bit bigger), and there should still only be one
output.

Integrated Circuits

Integrated circuits
accomplish such unique tasks, and are so numerous, that they don’t
really get a unique circuit symbol. Usually, an integrated circuit is
represented by a rectangle, with pins extending out of the sides. Each
pin should be labeled with both a number, and a function.

ATmega328, ATSHA204, and ATtiny45 IC symbols

Schematic symbols for an ATmega328 microcontroller (commonly found on Arduinos), an ATSHA204 encryption IC, and an ATtiny45 MCU. As you can see, these components greatly vary in size and pin-counts.

Because ICs have such a generic circuit symbol, the names, values and
labels become very important. Each IC should have a value precisely
identifying the name of the chip.

Unique ICs: Op Amps, Voltage Regulators

Some of the more common integrated circuits do get a unique circuit
symbol. You’ll usually see operation amplifiers laid out like below,
with 5 total terminals: a non-inverting input (+), inverting input (-),
output, and two power inputs.

Often, there will be two op amps
built into one IC package requiring only one pin for power and one for
ground, which is why the one on the right only has three pins.

Simple voltage regulators are usually three-terminal components with
input, output and ground (or adjust) pins. These usually take the shape
of a rectangle with pins on the left (input), right (output) and bottom
(ground/adjust).

Miscellany

Crystals and Resonators

Crystals or resonators are usually a critical part of microcontroller
circuits. They help provide a clock signal. Crystal symbols usually
have two terminals, while resonators, which add two capacitors to the
crystal, usually have three terminals.

Headers and Connectors

Whether it’s for providing power, or sending out information,
connectors are a requirement on most circuits. These symbols vary
depending on what the connector looks like, here’s a sampling:

Motors, Transformers, Speakers, and Relays

We’ll lump these together, since they (mostly) all make use of coils in some way. Transformers (not the more-than-meets-the-eye kind) usually involve two coils, butted up against each other, with a couple lines separating them:

Relays usually pair a coil with a switch:

Speakers and buzzers usually take a form similar to their real-life counterparts:

And motors generally involve an encircled “M”, sometimes with a bit more embellishment around the terminals:

Fuses and PTCs

Fuses and PTCs – devices which are generally used to limit large inrushes of current – each have their own unique symbol:

The PTC symbol is actually the generic symbol for a thermistor, a temperature-dependent resistor (notice the international resistor symbol in there?).

No doubt, there are many circuit symbols left off this list, but
those above should have you 90% literate in schematic reading. In
general, symbols should share a fair amount in common with the real-life
components they model. In addition to the symbol, each component on a
schematic should have a unique name and value, which further helps to
identify it.

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