A schematic is a map of a circuit showing the components and how
they're connected to one another.
You don't have to learn how to read a schematic to build the Pinscape
Controller projects, but it's a skill that might come in handy if you
need to troubleshoot one of the Pinscape boards after building it. So
this section provides a very quick introduction, hopefully just enough
that you can make sense of the Pinscape schematics should you ever
need to look at one.
Schematics use their own special symbolic language, the way that music
has its language of staffs and notes, so they can look pretty opaque
at first. A lot of electronics-for-newbies tutorials try to avoid the
formalism of schematics by using pictorial circuit diagrams. I've
even used a few of those in this guide, like this one from the
chapter, showing how to wire a
current-limiting resistor into a flasher LED circuit:
That's fine for simple circuits, but it doesn't "scale up" well to
large circuits with many components. It's also a bit fuzzy, in
that the little pictures of the parts could be mistaken for something
else, especially if there were more than a few distinct parts in the
diagram. Warm and fuzzy might be fine for artists and puppies, but
engineers don't like fuzzy. They like clarity and precision.
Schematics were invented as a more concise and precise way of showing
this kind of information. To a large extent, a schematic is a lot
like the pictographic representation above; the big difference is that
we replace the pictures of the parts with symbols representing the
parts. Here's a schematic version of the circuit above:
The symbols give schematics their precision, since there's a standard
set of them that everyone agrees on. They constitute a sort of
vocabulary, so you'll find yourself able to read most schematics
pretty readily once you learn the basic symbol set.
The simplest schematic symbol is a wire, which is shown as a line
between two parts.
This schematic idea of a "wire" is an abstraction - it doesn't
necessarily represent a literal piece of wire. And you'll notice it
doesn't say anything about how long the wire is or where it goes on
the circuit board. A schematic "wire" just represents some
type of electrical connection. In the physical realization of the
circuit, the schematic "wire" could be an actual piece of wire, or it
could be a copper trace on a printed circuit board. The schematic
wire is just saying that the two points are connected electrically,
with the details left up to whoever builds the circuit physically.
(Which also means that you could build a functionally identical
circuit in different physical arrangements: on a printed circuit board,
on a breadboard, with a bunch of loose parts and wires...)
When wires run directly into components as shown above, it means that
the components are connected to the wires. When wires cross over each other,
though, they're not automatically connected. Here are two
separate pieces of wire that aren't electrically connected to each
Two separate wires, not connected to each other
Some schematics show non-connected wire crossings with a little hump:
Another way of showing a non-connected wire crossing
The little hump is to make it more explicit that there's no
connection. But this seems to be more of a "beginner" convention that
you don't see much in engineering schematics. The modern practice is
to use simple straight crossings.
Whenever the wires at a crossing are connected, we add a big
dot to indicate the connection:
Two wires connected together
The dot is also used at any connected "T" junction:
When a schematic reaches a certain level of complexity, you get so
many wire lines going across such large swaths of the diagram that it
gets hard to follow them all. So there's a convention that greatly
reduces the tangle of wire lines by removing long lines and replacing
them with "cross-references".
A cross reference point looks like this:
The word inside the arrow-shaped box is the cross-reference label.
Sometimes you see it with the arrow box, and sometimes you see it with
just the text label:
They both mean the same thing. I like the box notation because it
makes it easier to spot these points at a glance, but some engineers
don't bother with them. It's like the little humps to represent
explicit "not connected" crossings: a text label by itself at the end
of a wire can only mean one thing, so some engineers see the little
box as redundant.
In either case, what this means is that this point in the wiring is
connected to all of the other points in the wiring that have
the same label. This lets you connect two points on opposite sides of
the schematic without having to route a green line all the way across
Now, that's just an oversimplified example - when the two "CLK" points
are close together like that, you'd usually just draw the wire. But
imagine if those two "CLK" points were at opposite corners of a large
schematic. In that case, drawing the wire all the way across would be
harder to trace than the cross-reference notation.
The label in a cross-reference is just an arbitrary name defined by
the person who drew the schematic. It doesn't mean anything within
the schematic language the way that a resistor symbol means something;
it's just a name for that connection point, like the name of a street.
These names are usually chosen to be somehow descriptive, but that's
purely within the context of the particular circuit.
In the Pinscape expansion board schematics, you see this in a lot of
the connections to the KL25Z:
This notation is necessary for many of the KL25Z connections because
it's such a central component with so many things connected. Many of
the connected items are in far-flung areas of the schematic. It would
have been too hard to follow all of those green wire lines across the
whole page - or worse still, across several pages.
In addition to the cross-reference notation, there's another wiring
short-hand known as "buses" that the Pinscape schematics use in
places. A bus represents a bundle of related wires all grouped into
one line on the map.
On the old Williams pinball schematics, buses are represented like
The thick striped wire is the bus, which represents the combination of
all of those individual D0 through D7 wires going into it at the top.
This bus might run between a couple of IC chips, or it might connect
many parts. At other points along the bus, some or all of those same
D0 through D7 wires would come back out of it, representing the
break-out into the individual connections again.
The EAGLE notation for a bus is a little different. I personally
prefer the old Williams notation, since I think it's clearer, but the
EAGLE approach is really the same idea once you learn what it looks
like. EAGLE's way of drawing this is to just use a thick blue line to
represent the bus.
Some important things to note about the bus notation:
- Unlike regular wire-to-wire connections, there are no "dots" to
indicate the connection points. Any regular wire that ends at a bus
line is connected to the bus.
- The wires going into the bus aren't connected to each other.
D0 through D7 are still all separate wires. This is just
short-hand to show the whole group of wires as a single line/bar
rather than having to draw all of them individually.
- If you want a physical analogy, you can think of the bus as a
shrink-wrap tube that wraps around all of the wires making up
"Ground" has several meanings in electronics, so you see different
symbols for it. The most common symbol you see is this, which
typically represents an "Earth" ground, meaning literally a connection
to the soil, usually through the ground prong in your house's power
In the EAGLE schematics, we don't use that exact symbol, and we don't
have any points where we're talking about the literal Earth ground.
You'll see these two symbols in the EAGLE schematics instead:
In our schematics, these are what's known as DC grounds. If you think
about a power supply as though it were a battery, it would have a "+"
post and a "-" post. In that way of thinking, the "-" post
corresponds to the DC ground. That's not quite the way engineers
think of power supplies, though: they think of what you'd call the "-"
end of the battery as 0V for "zero volts". That's the reference point,
and all of the other supply voltages are relative to that reference
point - so the disk connectors on an ATX power supply, for example,
have a +5V supply line and a +12V supply line, relative to that 0V.
This 0V point is what we call "ground" in a DC circuit.
Why do we have two different ground symbols, and what's "GND3"? I'm
sure you already guessed that "GND" is an abbreviation for "Ground".
"GND3" stands for "Ground 3", which is a separate DC ground point in
the circuit from the regular "GND". You'll see "GND2" in other
places, which is a second one.
The Pinscape schematics use the multiple grounds for two reasons.
- The first is what you might expect, which is to isolate different
parts of the circuit. The expansion boards are set up to isolate the
"logic" part from the "power" part, by using separate power supplies
for the two sections. The regular "GND" point is the 0V ground
connection for the PC power supply (the "logic" section), and "GND1"
is the corresponding connection for the secondary power supply that
powers your knocker coil and shaker motor (the "power" section).
We use the two separate symbols to suggest this separation visually.
- The second is an inelegant way handling some special needs of the
circuit boards. GND1, GND2, and GND3 in these schematics are actually
all connected together. They're given separate names because that
lets us persuade EAGLE to given them different trace widths on
the circuit boards, mostly so that some of the connections can
handle high current loads.
All of the connected ground points use the same "triangle" symbol,
which hopefully helps suggest the connection visually.
In all of these cases, the GND points are ultimately connected to the
Ground connection on a power supply unit. For a PC-style ATX power
supply, the "ground" connection is the black wire in all of the disk
cables coming out of the unit.
As with the grounds, the expansion boards use two symbols to represent
power supply connections:
We use the two symbols for the same reason that we do with the
grounds: because the expansion boards are designed to be connected to
two separate power supplies. One symbol, the little arrow, represents
the main PC power supply. We use the circle-plus symbol for the
secondary power supply.
The power supply connections are labeled with the voltage.
Just to be clear, these power supply symbols represent power
inputs, where you connect the circuit boards to a separate
power supply unit that supplies the labeled voltage. (As opposed to
representing power outputs where the boards are generating power for
something else. That's not something we do in any of the Pinscape
A resistor is a simple component that adds electrical resistance
(analogous to friction in a mechanical system) to a circuit.
The symbol on a schematic is a jagged line.
The version on the right is the same as the version on the left, just
rotated 90°. We wanted to show that just to clarify that it means
the same thing no matter how it's rotated. The same is always true
for all other component types. Schematic writers will orient each
symbol as they see fit for legibility.
A resistor on a schematic is usually accompanied by two labels,
usually placed on either side of the resistor symbol.
The first is an "R" followed by a number - in the example above, R13
and R14. This is formally called the "reference designator" for the resistor,
or just the "designator". It's an arbitrary, unique identifier for
the part, primarily for cross-referencing to the parts list. It has
no meaning by itself; it's just a name. The "Rnumber" notation
is just a convention, too; in principle any sort of label would do.
But the "R" labeling for resistors is practically always used.
Designators always have to be unique throughout the schematic, so that
you can identify each individual physical part.
The second label is the resistance value in Ohms. This is usually
written in one of these formats:
- 47R means 47 Ohms - the "R" suffix is usually
used instead of the real symbol for Ohms, Ω, probably
because the Ω symbol could be mistaken for a zero, or
maybe just because it was hard to enter the Ω symbol in
- 4R7 means 4.7 Ohms - an "R" sandwiched between
numbers like this stands in for a decimal point; this notation
is used because real decimal points aren't always legible in
crowded areas or tiny fonts
- 47K means 47 Kilo Ohms = 47 kΩ = 47000 Ohms;
the "K" means "times a thousand Ohms"
- 4K7 means 4.7 Kilo Ohms = 4.7 kΩ = 4700 Ohms;
as with the embedded "R", an embedded "K" replaces a decimal point,
and also still means "times a thousand Ohms"
- 47M means 47 Mega Ohms = 47 MΩ = 47,000,000 Ohms;
"M" means "times a million
- 4M7 means, you guessed it, 4.7 Mega Ohms = 4.7 MΩ = 4,700,000 Ohm
A resistor has two connections to the outside world. The schematic
symbol shows this as a straight line sticking out of each end.
Resistors aren't polarized, meaning the two ends are interchangeable.
There's nothing in the symbol indicating which way the resistor goes
because it doesn't matter which way it goes.
A capacitor is a simple component that adds electrical capacitance to
a circuit, which is similar to a (very) tiny rechargeable battery.
The symbol for a capacitor consists of two parallel lines
separated by a small gap, or one straight line and one curved line
next to each other. In some cases, there might be a little "+" sign
adjacent to the straight line.
As with resistors, each capacitor in a schematic is typically
accompanied by two labels.
The first label is a "C" followed by a number. This is the
capacitor's reference designator - an arbitrary ID for the part,
purely for looking it up in the parts list. It's the capacitor
equivalent of the "R" number for a resistor. It doesn't have any
meaning by itself; it's just a name to look up in the parts list.
Reference designators always have to be unique throughout the whole
schematic, so that you can uniquely identify every physical part that
goes into the circuit. Note that there's no absolute rule that a
capacitor's designator has to start with "C", but almost everyone uses
that convention, so it's practically a rule.
The second label is the capacitance value in Farads. This is almost
always in one of the following formats:
- 100pF means 100 pico Farads or 100 trillionths of a Farad
- 100nF means 100 nano Farads or 100 billionths of a Farad
- 100uF means 100 micro Farads or 100 millionths of a Farad
(this is more properly written 100µF, but the Roman alphabet
"u" is usually used instead because of pervasive ASCII chauvinism in
- 100mF means 100 milli Farads or 100 thousandths of a
Farad (these are extremely large capacitors that you rarely see in
micro-electronics, but you might see one in a power supply; there's
a 30mF capacitor in my Whilrlwind's lamp power supply circuit,
and it's about the size of a soda can)
A capacitor has two connections, represented in the symbol by the
lines coming out of either end.
If there's a "+" sign in the symbol, the capacitor is a "polarized"
type, meaning that one end has to be connected to the positive voltage
and the other end is for the negative voltage. The "+" sign in the
symbol marks the end that connects to the positive voltage.
If there's no "+" sign in the symbol, the capacitor is an
"unpolarized" type, meaning it doesn't matter which end connects to
which voltage. The two ends are interchangeable (like in a resistor).
The polarized or unpolarized status is a function of the physical type
of capacitor you're using. If the schematic symbol has the "+" sign,
you must use a polarized capacitor in the physical build. If
not, you must use an unpolarized capacitor. You can generally
tell if a particular physical capacitor is polarized by looking at its
- A ceramic disc capacitor is always unpolarized
- An electrolytic capacitor is always polarized
There are several other types besides these, but these are the only
types you'll see in the Pinscape boards. Most of the other, more
exotic types are non-polarized, including film and glass capacitors.
Tantalum capacitors are a type of electrolytic capacitor, so they
A diode is a semiconductor that only lets current flow in one
direction, sort of an electronic one-way valve. See Diodes
The symbol for a diode on a schematic is an arrow with a bar:
Each diode on a schematic is typically accompanied by two labels. The
first is a "Dnumber" label giving the reference designator, for
looking up in the parts list. As with resistor "R" numbers and
capacitor "C" numbers, this has no meaning by itself; it's just an
arbitrary ID for cross-referencing with the parts list. Almost
everyone uses "D" for "diode" in these labels by convention.
The other label is the semiconductor identifier for the type of
diode to be used. This is sort of like a manufacturer part number or
catalog number, but it's not specific to any one manufacturer; it's a
generic descriptor system that the industry uses. Diodes don't have a
simple "unit" that describes them like Ohms for resistors or Farads
for capacitors, so schematic writers use this semiconductor ID to
specify which part they want you to use. For a diode, this usually
starts with "1N", as in the example above, 1N4007. You can use
this ID on sites like Mouser to search for matching parts to buy.
Diodes are inherently polarized, so they have to be wired into the
circuit in the correct direction. If you put a diode in backwards, it
won't work properly (and might do damage). The direction is indicated
by which way the arrow is pointing. On the physical diode, you should
see a stripe painted on one end; that stripe corresponds to the bar
that the arrow is pointing to in the schematic symbol.
An LED is actually just a special case of diode. That's the "D" in
the acryonym - "Light Emitting Diode" - and it's quite literal. The
schematic symbol for an LED is therefore basically the same as the
symbol for a regular diode, with an embellishment to indicate that
it's the special light-up kind: a couple of little arrows representing
the photons flying away.
An "LEDnumber" reference designator usually takes the place of
the "D" designator for a regular diode, but there's less of a
universal convention about this, so you might see other formats. You
should always see some designator, though, for looking up in the parts
And as with a regular diode, an LED symbol will often be accompanied
by some sort of formal part ID, such as a manufacturer part number, to
tell you what to buy. This might not be present in the schematic,
though, in which case you'll have to check the parts list.
Transistors - bipolar
A bipolar transistor (or bipolar junction transistor, BJT) is a common
type of transistor that's used in all sorts of circuitry as a small
amplifier or an electronic switch.
The symbol for a transistor consists of a thick bar with three lines
sticking out, one straight line on one side, and two diagonal lines on
the other side. One of the diagonal lines has an arrow, which might
point towards or away from the middle bar.
If the little arrow points away from the bar, the symbol
represents an "NPN" transistor. If the arrow point towards the
bar, it's a "PNP" transistor.
Note that the little arrow might be shown at top or bottom, and it
might be on the left side or the right side. None of that makes any
difference - the symbol means the same thing no matter how it's
flipper or rotated. Schematic writers will flip the symbol
top-to-bottom, or left-to-right, or rotate it at different angles,
according to what's convenient to make the lines between nearby
connections shorter. It doesn't change the meaning.
The three lines represent the three connections to the transistor,
called the base, collector, and emitter:
- The straight line by itself on one side is always the base or B
- The diagonal line with the arrow is always the emitter or E
- The other diagonal line is always the collector or C
On some schematics, the whole thing will be enclosed in a circle:
The circle doesn't change anything; it's just an alternative way
of drawing the symbol.
Transistors have parts list tags just like other components. These
most commonly start with "T" or "Q". As with the "R" tags for
resistors and "C" tags for capacitors, these are just arbitrary tags
to look up in the parts list, with no other meaning.
Transistors are also usually labeled with the semiconductor ID, like a
diode is. In the case of a transistor, this usually starts with "2N".
You might also see other part numbers, such as the "BC337" in the
examples above. When two numbers are listed for one part like this,
it indicates alternative parts that you can use - so in the
case of T8 above, you could use a 2N4401 or BC337 interchangeably.
Transistors have to be inserted into the circuit with the three prongs
in exactly the right order. As with diodes, each prong has a
different function, and the part won't work if it's not inserted
correctly. There's no standard way of marking a physical transistor
to indicate which leg is which - the only way to tell is to look it up
in the manufacturer's data sheet. In the case of the Pinscape
expansion boards, though, you can tell how to orient the part from the
looking at the silk-screened markings on the circuit board; we'll
explain that in Transistors
Transistors - Darlington
A Darlington transistor is a variation on the basic bipolar transistor
that combines two bipolar transistors in one physical package, for
greater amplification and power handling than a regular bipolar
transistor can handle. See "Darlingtons" in
For the purposes of building the Pinscape boards, Darlingtons are the
same in every respect as bipolars. But they have a different symbol
in a schematic, so we wanted to show you what that looks like so that
you can recognize it when you see it:
The symbol is pretty literal - it looks like two regular transistors
connected together, because that's just what a Darlington is. A
Darlington still has the same three external connections (base,
collector, and emitter).
Transistors - MOSFET
A MOSFET is another kind of transistor constructed in a different way
from a bipolar transistor. It performs the same transistor functions
as a bipolar, but the electrical characteristics are somewhat
different, so it has its own representation on a schematic:
As with bipolars, there are two types of MOSFETs, known as N-channel
and P-channel MOSFETs. The schematic symbols for the two types are
almost the same, befitting their similar construction and behavior,
with one subtle difference: the direction the arrow points in the
middle of the diagram. In an N-channel MOSFET, the arrow points
inwards, into the middle section; in a P-channel MOSFET, it points
MOSFET symbols in a schematic are labeled like other transistors, with
a reference designator (we're using a Q prefix here, but you
might also see a T prefix) and a part number. For MOSFETs,
this is almost always a manufacturer part number, so there won't be
any particular pattern to it; it'll just be an alphanumeric string
that you can look up on Mouser and in other vendor catalogs.
Like bipolar transistors, MOSFETs have three prongs with distinct
functions, and they have to be oriented properly when installed.
The prongs of a MOSFET go by different names from a bipolar's legs:
- The Gate is the prong off by itself on one side
- The Source is the prong that connects to the arrow
- The Drain is the remaining prong
As with bipolar transistors, there's no standard marking system to
identify which leg is which on the physical part; you just have to
look it up in the manufacturer's data sheet. The Pinscape expansion
boards show how the part is oriented on the silk-screened markings
on the circuit boards.
Integrated Circuit (IC) chips are complex devices consisting of many
components packed into a single package. See IC Chips
ICs are extremely diverse in function and physical packaging, so it's
not entirely fair to lump them all into a single category. But there
are enough commonalities to how they're handled in schematics that we
can make some practical generalizations. For our purposes, an IC is a
bit of circuitry all packed into a discrete physical package, with
multiple connection points ("pins" or "leads" coming out of the
physical chip). The schematic treats an IC as a "black box": a bunch
of wires connect it to the outside world, but what's inside is of no
concern in the schematic. As a result, the schematic symbols for
ICs look pretty much like empty boxes:
Here are some features to note:
- The overall IC package is represented by a rectangular box
- We're using the term "black box" figuratively, as you can see that
we haven't literally drawn the box in black ink here; "black box" is a
metaphor that engineers use to talk about something with complex inner
workings that we don't have to see (or understand) in order to use it
- The wires coming out of the box represent the "pins" or
"leads" on the physical IC, which are the electrical connection points
- Different ICs have different numbers of pins, so you might see boxes
like this with three wires coming out (such as for a voltage
regulator), or a couple hundred wires (for a CPU chip), or anything in
- The positions of the wires around the perimeter of the box don't
correspond to the physical layout; this is just an abstract
representation, like any other schematic symbol
- The order of the wires in the symbol doesn't reflect the ordering of
the pins on the physical chip - for that you need to consult the the
little number written adjacent to each wire just outside the box,
which tells you the pin number on the physical chip that this wire
- The labels written on the inside of the box adjacent to
the pins are mnemonics for the functions of the pins; these are
purely for convenience, to help you remember the function of each
pin without having to keep going back to the chip's data sheet
- The schematic symbol will usually be accompanied by a reference
designator, analogous to "R5" for a resistor or "C7" for a diode; for
an IC, it's usually of the form "IC10", but lots of other prefixes are
used, including odd ones like "U$" - engineers started running out of
unique letters for these tags at some point, so they resorted to other
symbols. The prefix might also be specific to the type of IC; for
example, the Pinscape schematics use "OK" for optocouplers.
- The schematic symbol will also usually be accompanied by the
manufacturer part number for the specific IC ("TLC5940" in the case of
this example); some of these are generic part descriptors for parts
made by many manufacturers, while others are manufacturer-specific
There are exceptions to this "black box" treatment. Some types of ICs
have specific functions that are so commonly used that they have their
own unique schematic symbol that's more representative of the
function. We'll see this for optocouplers below. Other examples
include common logic gates, such as NAND and NOR gates and inverters,
which sometimes (but not always) are shown with special logic symbols
in place of the plain box. The Pinscape boards don't use any of
those types of symbols, but you might see them in other schematics.
For example, a NAND gate might be drawn like this:
That's a very specific notation that engineers recognize as a NAND
gate, so it's sometimes used in place of the more generic "black box"
notation for miscellaneous ICs. But you might just as well see the
plain black box notation; it's really up the schematic writer. You
might even see a hybrid notation that shows the NAND IC as a black
box, and then also draws the logic symbol inside the box. This
is just a more pictographic equivalent of the little mnemonic labels
that you see in the TLC5940 diagram above, since it shows you the
function of each pin visually. For example, here's how you might
see a chip that contains four NAND gates represented:
There's another "advanced" convention that you should know about
when it comes to IC chips, and even some other types of components
(such as relays, as we'll see below).
Some chips come with two or four or eight copies of the same basic
building block. For example, the Pinscape boards use the PC847 chip,
which consists of four separate optocouplers on one chip. That
makes it a "quad optocoupler" chip. Pinscape also uses a "quad
Darlington" chip, the ULN2064, which consists of four Darlington
transistors on one chip. This is quite common with basic components
like logic gates, optocouplers, op-amps, and transistors.
In schematics, one way to represent these multi-gang chips is the
generic way we saw above, where you draw a big box for the entire
chip. The ULN2064 uses a generic symbol like that in the Pinscape
So nothing new so far! But now we come to the novel part. Sometimes,
rather than using the generic black box format, schematics will
represent a multi-gang part with its individual building blocks all
separated from one another, as though they were separate
components. EAGLE uses this approach for the PC847, that
"quad optocoupler" we mentioned. Rather than drawing it as a big
box with 16 pins coming out of it - which is, in fact, exactly
what the physical package looks like - EAGLE draws this
as though it were four separate optocouplers. Here's an excerpt
from the Pinscape "main interface board" showing one PC847 broken
up into four optocoupler symbols:
If you didn't already see what we meant about how schematic symbols
are "abstract", this probably really drives it home.
Two questions: Why in the world do they do this? And how are you
supposed to tell that those four boxes are really one physical IC
First the "why". They do this to make the schematics more readable.
I know, it can seem like it does the opposite. But if you think about
it in terms of understanding what the circuit does as opposed
to how to build it out of parts, this representation is actually a lot
more useful than drawing all of those 16 wires going into a black box.
With this format, you can plainly see which wires control the LEDs and
which wires are connected to the photo-transistors.
There's another benefit that's not even apparent in this picture, too.
Those four boxes representing the individual optos don't have to be
grouped together in one place in the schematic - they can be split up
and spread out. They really are four separate boxes as far as the
schematic is concerned. This allows the schematic writer to place
each one close to the other components it's connected to, so that the
wire connections are shorter and easier to follow. The Pinscape
boards keep all of the groups like this together, but if you look at
some old Williams pinball schematics, you'll find that they take
ruthless advantage of this ability to spread the parts around. You'll
find quad NAND gate chips with the individual gates on different
pages, and dual op-amp chips with the individual op-amp blocks
likewise widely separated.
Now to the second question, how you're supposed to relate the four
boxes back to one physical chip. The trick is to look for matching
tags. You can see that each individual opto in the diagram above has
its own separate set of labels - each one is tagged "PC847" (the chip
name) and "OK1x" (the reference designator for the parts list lookup)
- as though it were a standalone part. The designator is what gives
away the secret that all of these "OK1" elements are part of the same
physical chip. And why is that? Because a designator is
always unique across the whole schematic - like Highlander,
there can be only one OK1. The fact that the same designator appears
on four symbols means that the symbols are all portions of the same
Okay, back up a sec... I'm sure you noticed that these aren't
actually all labeled "OK1". They're labeled OK1A, OK1B, OK1C, and
OK1D. But when have we ever seen a letter after a number in a
designator before? Never. Tags until now always ended in a number.
You've probably already guessed what it means when you add a letter to
the end: it means that we're talking about a sub-block of a multi-gang
chip like this. The physical chip is still called "OK1", but they've
given these additional "A" through "D" suffixes to the individual
optos within OK1 to help us distinguish them.
Those A-B-C-D suffixes aren't always used, by the way. They're used
for this particular opto, but for other types of chips, you might just
see the same base designator repeated on each block. Each block might
be tagged, say, IC9, with no suffix. The A-B-C-D suffixes aren't
really all that necessary, since you can still tell which block is
which in physical terms by looking at the pin numbers. If you look at
OK1A through OK1D above, you'll see that each pin is still numbered in
terms of the overall 16 pins of the physical chip, the same as if it
were the black-box kind of symbol.
An optocoupler is a special kind of IC chip that connects two parts of
a circuit via a photo-emitter and a photo-receiver. This provides
electrical isolation between the two parts of the circuit while
allowing them to transmit information across the interface. See
"Optocouplers" in IC Chips
As we mentioned above, some types of IC chips are so commonly used
that they get their own schematic symbols. The optocoupler is one of
these special cases. An optocoupler looks like this on our schematics:
That's a little like the NAND gate example we talked about above, in
that it starts with the generic IC "black box", but then adds a
pictograph inside to depict what the pins do. An optocoupler
internally consists of an LED (usually infrared) and a phototransistor
(a special type of transistor that's activated when light hits it,
rather than being controlled by an electrical signal), so you see the
symbols for an LED and a transistor. Just as with the mnemonic text
labels inside the black box on our TLC5940 example earlier, the
pictogram is just a mnemonic to help you remember what the pins do.
In terms of actually using the chip physically, you can really ignore
all of that, since all you have to pay attention to is the pin numbers
written on the outside wires - exactly like any generic IC chip.
Note one other deviation from more generic ICs: the Pinscape
schematics use "OK" as the designator prefix instead of "IC", so in
this case, "OK5" instead of "IC5". You might also see "OC" prefixes
in other people's schematics.
A relay is an electrically-controlled mechanical switch. An
electromagnet in the relay operates a little rocker switch that
connects and disconnects another circuit. See Relays
The schematic symbols for a relay vary. Here's the format that we use
in the EAGLE plans for the Pinscape boards:
This is a little hard to parse, because it breaks up the relay into
its component parts:
- the little box at the right represents the electromagnet
- the two clusters at the left represent the mechanical switches
(this particular relay has two of them, because it's a "double pole"
relay, meaning it has two electrically independent switches operated
by the same electromagnet)
This follows exactly the same convention that we saw for some IC chips
with multiple repeated blocks - see "Multi-gang chips"
for more on that. The short
summary is that we can tell that these three little blocks are
actually part of the same physical relay from their tags. They're all
tagged "K1". Since a designator must be unique across the whole
schematic, the fact that we have three things tagged K1 can only mean
that they're all part of the same physical component.
As with an IC chip, the numbers on the connection points indicate the
pin numbers on the physical relay. There aren't really any
conventions for how the pins on a relay are numbered; it's just
something you have to look up in the data sheet for the individual
You might also see schematic symbols for relays that are more literal,
with a pictograph for the electromagnet coil, and the whole thing
enclosed in a black box like an IC chip. For example:
Connectors and pin headers
Circuit boards need connections to the outside world, usually in the
form of some kind of plug-in connector. We provide an overview of
some of the most common types, and the ones we use on the Pinscape
boards, in Connectors
On a schematic, we draw connectors like this:
It looks a bit like a generic IC, but note that the wires all go into
the box and connect to little circles. The circles represent the pins
on the connector. The numbers next to the circles are the pin
numbers, which tell you which pins they correspond to on the physical
connector. The pin numbering conventions are different for different
parts; we explain our conventions in Connectors
always with schematic symbols, the order and arrangement of pins shown
in the schematic doesn't necessarily correspond to the physical pin
layout, so you have to pay attention to the pin numbers.
On the Pinscape schematics, most connectors have a "JP" label, for
"jumper", as in JP7 or JP15. (Another common convention that you'll
see on other schematics is a simple "J" prefix, such as J9.) As
usual, this is the reference designator, for looking up the connector
in the parts list, and (as usual) it has no meaning other than to
serve as a cross-reference. You might also see a manufacturer part
number, as in the example above. Some of the connector types are
generic enough that you can substitute equivalent parts from other
manufacturers, so the part number might only be a suggestion to help
you find a matching part. It's always critical to match the total
number of pins when substituting parts.
One subtle detail to note in the diagram above is that some of the
pins might be left unconnected. That's indicated by the simple
absence of a wire connected to the pin, as in pin #16 in the
You might sometimes see one (or possibly more) of the pins drawn with
an "X" over it:
This means that the marked pin is meant to be snipped off on the
physical pin header, and the same pin socket in the mating connector
plug is meant to be blocked (literally plugged up with a little piece
of plastic, so that you couldn't insert a pin there if you wanted to).
The point is to "key" the connector so that it's impossible to insert
the wrong way. When you connect the plug the right way, the blocked
socket in the plug lines up with the snipped-off pin on the header, so it
fits and everyone's happy. If you try to insert it the wrong way, the
blocked socket collides with one of the pins that wasn't snipped,
preventing you from attaching it that way and alerting you that you've
got something wrong.
The Pinscape schematics don't use keyed connectors like that anywhere,
but it's something you might see on other schematics. The Williams
pinball machines do this for most of their connectors to help prevent
operators from re-connecting cables the wrong way when making repairs.
Pin numbering on the physical connector
The pin numbers on the schematic symbol tell you which physical pin
corresponds to each schematic pin.
Single-row pin connectors: the physical pins are numbered
sequentially starting at one end. On many boards, such as the KL25Z,
they indicate this by printing a "1" next to pin 1:
You can then infer all of the other pin numbers by just counting them
across the row, starting at pin 1. Note that in the picture above,
they've also helpfully labeled pin 3 at the other end. The KL25Z does
this on each header, labeling the pins at either end.
For the Pinscape boards, the convention is to show a little triangular
arrow next to pin 1:
Pin 1 is always at the end with the arrow, and the rest of pins are
numbered sequentially across the row (2, 3, 4...).
Double-row pin connectors: As with the single-row headers, look
for a pin 1 marking to identify pin 1. Some boards (including the
KL25Z) mark this with a numeral "1" next to one of the pins. The
Pinscape boards use the same triangular arrow they use for single-row
For double-row headers, the numbering goes by column: