A schematic is a visual representation of a circuit. As such, its
purpose is to communicate a circuit to someone else. A schematic in a
special computer program for that purpose is also a machine-readable
description of the circuit. This use is easy to judge in absolute terms.
Either the proper formal rules for describing the circuit are followed and
the circuit is correctly defined or it isn't. Since there are hard rules
for that and the result can be judged by machine, this isn't the point of
the discussion here. This discussion is about rules, guidelines, and
suggestions for good schematics for the first purpose, which is to
communicate a circuit to a human. Good and bad will be
judged here in that context.
Since a schematic is to communicate information, a good schematic does
this quickly, clearly, and with a low chance of misunderstanding. It is
necessary but far from sufficient for a schematic to be correct. If a
schematic is likely to mislead a human observer, it is a bad schematic
whether you can eventually show that after due deciphering it was in fact
correct. The point is clarity. A technically correct but
obfuscated schematic is still a bad schematic.
There can be different opinions even among professionals in some
areas. However, these tend to be on the finer points. You will find that
there is broad agreement among those that create and read schematics
regularly. Here are the rules:
Use component designators
This is pretty much automatic with any schematic capture program. If
you draw your schematic on a napkin and then scan it, make sure to add
component designators. These make the circuit much easier to talk about.
I have skipped over questions when schematics didn't have component
designators because I didn't feel like bothering with the second 10
kΩ resistor from the left by the top pushbutton. It's a lot
easier to say R1, C5, Q7, etc.
Clean up text placement
Schematic programs generally plunk down part names and values based
on a generic part definition. This means they often end up in
inconvenient places in the schematic when other parts are placed nearby.
Fix it. That's part of the job of drawing a schematic.
Some schematic capture programs make this easier than others. In
Eagle for example, there can only be one symbol for a part. Some parts
are commonly placed in different orientations, horizontal and vertical
in the case of resistors. Diodes can be placed in at least 4
orientations since they have direction too. The placement of text around
a part, like the component designator and value, probably won't work in
other orientations than the symbol was originally drawn in. If you
rotate a stock part, move the text around afterward so that it is easily
readable, clearly belongs to that part, and doesn't collide with other
parts of the drawing. Vertical text looks stupid and makes the
schematic hard to read.
I make separate redundant parts in Eagle that differ only in the
symbol orientation and therefore the text placement. That's more work
up front, but makes it easier when drawing a schematic. However, it
doesn't matter how you achieve a neat and clear end result, only that
you do. There is no excuse.
Sometimes we hear whines like "But CircuitBarf 0.1 doesn't let me
do that". So get something that does. Besides, CircuitBarf 0.1
probably does let you do it, just that you were too lazy to read the
manual to learn how and too sloppy to care. Draw it (neatly!) on paper
and scan it if you have to. Again, there is no excuse.
For example, here are some parts at different orientations. Note how
the text is in different places relative to parts to make things neat
Don't let this happen to you:
This is a small snippet of what someone actually dumped on an
Electrical Enginnering Q&A site.
Consider how a teacher would respond to this mess if handed in as
homework, what a prospective employer would think if they found this
from you out on the 'net, or even what volunteers on a site like this
would think when deciding whether a question is worth their free time to
Basic layout and flow
In general, it is good to put higher voltages towards the top, lower
voltages towards the bottom, and logical flow left to right. That's
clearly not possible all the time, but at least a generally higher level
effort to do this will greatly illuminate the circuit to those reading
One notable exception to this is feedback signals. By their very
nature, they feed "back" from downstream to upstream, so they
should be shown sending information opposite of the main flow.
Power connections should go up to positive voltages and down to
negative voltages. Don't do this:
There wasn't room to show the line going down to ground because other
stuff was already there. Move it. You made the mess, you can unmake
it. There is always a way.
Following these rules causes common subcircuits to be drawn similarly
most of the time. Once you get more experience looking at schematics,
these will pop out at you and you will appreciate this. If stuff is
drawn every which way, then these common circuits will look visually
different every time and it will take others longer to understand your
schematic. What's this mess, for example?
After some deciphering, you realize "Oh, it's a common emitter
amplifier. Why didn't that ##### just draw it like one in the first
Draw pins according to function
Show pins of ICs in a position relevant to their function, NOT HOW
THEY HAPPEN TO STICK OUT OF THE CHIP. Try to put positive power pins at
the top, negative power pins (usually grounds) at the bottom, inputs at
left, and outputs at right. Note that this fits with the general
schematic layout as described above.
Of course, this isn't always reasonable and possible. General-purpose
parts like microcontrollers and FPGAs have pins that can be input and
output depending on use and can even vary at run time. At least you can
put the dedicated power and ground pins at top and bottom, and possibly
group together any closely related pins with dedicated functions, like
crystal driver connections.
ICs with pins in physical pin order are difficult to understand. Some
people use the excuse that this aids in debugging, but with a little
thought you can see that's not true. When you want to look at something
with a scope, which question is more common "I want to look at the
clock, what pin is that?" or "I want to look at pin 5, what
function is that?". In some rare cases, you might want to go around
an IC and look at all the pins, but the first question is by far more
Physical pin order layouts obfuscate the circuit and make
debugging more difficult. Don't do it.
Direct connections, within reason
Spend some time with placement reducing wire crossings and the like.
The recurring theme here is clarity. Of course, drawing a direct
connection line isn't always possible or reasonable. Obviously, it can't
be done with multiple sheets, and a messy rats nest of wires is worse
than a few carefully chosen "air wires".
It is impossible to come up with a universal rule here, but if you
constantly think of the mythical person looking over your shoulder
trying to understand the circuit from the schematic you are drawing,
you'll probably do alright. You should be trying to help people
understand the circuit easily, not make them figure it out despite the
Design for regular size paper
The days of electrical engineers having drafting tables and being set
up to work with D size drawings are long gone. Most people only have
access to regular page-size printers, like for 8 1/2 x 11-inch paper
here in the US. The exact size is a little different all around the
world, but they are all roughly what you can easily hold in front of you
or place on your desk.
There is a reason this size evolved as a standard. Handling larger
paper is a hassle. There isn't room on the desk, it ends up overlapping
the keyboard, pushes things off your desk when you move it, etc.
The point is to design your schematic so that individual sheets are
nicely readable on a single normal page, and on the screen at about the
same size. Currently, the largest common screen size is 1920 x 1080.
Having to scroll a page at that resolution to see necessary detail is
If that means using more pages, go ahead. You can flip pages back
and forth with a single button press in Acrobat Reader. Flipping pages
is preferable to panning a large drawing or dealing with outsized paper.
One normal page at reasonable detail is also a good size to show a
subcircuit. Think of pages in schematics like paragraphs in a
narrative. Breaking a schematic into individually labeled sections by
pages can actually help readability if done right. For example, you
might have a page for the power input section, the immediate
microcontroller connections, the analog inputs, the H bridge drive power
outputs, the ethernet interface, etc. It's actually useful to break up
the schematic this way even if it had nothing to do with drawing size.
Here is a small section of a schematic I received. This is from a
screenshot displaying a single page of the schematic maximized in
Acrobat Reader on a 1920 x 1200 screen.
In this case, I was being paid in part to look at this schematic so I
put up with it, although I probably used more time and therefore charged
the customer more money than if the schematic had been easier to work
with. If this was from someone looking for free help like on this web
the site, I would have thought to myself screw this and gone on
to answer someone else's question.
Label key nets
Schematic capture programs generally let you give nets nicely
readable names. All nets probably have names inside the software, just
that they default to some gobbledygook unless you explicitly set them.
If a net is broken up into visually unconnected segments, then you
absolutely have to let people know the two seemingly disconnected nets
are really the same. Different packages have different built-in ways to
show that. Use whatever works with the software you have, but in any
case, give the net a name and show that name at each separately drawn
segment. Think of that as the lowest common denominator for using "air
wires" in a schematic.
If your software supports it and you think it helps with clarity, by
all means, use little "jump point" markers or whatever. Sometimes these
even give you the sheet and coordinates of one or more corresponding
jump points. That's all great but label any such net anyway.
The important point is that the little name strings for these nets
are derived automatically from the internal net name by the software.
Never draw them manually as arbitrary text that the software doesn't
understand as the net name. If separate sections of the net ever get
disconnected or separately renamed by accident, the software will
automatically show this since the name shown comes from the actual net
name, not something you typed in separately. This is a lot like a
variable in a computer language. You know that multiple uses of the
variable symbol refer to the same variable.
Another good reason for net names is short comments. I sometimes
name and then show the names of nets only to give a quick idea what the
purpose of that net is. For example, seeing that a net is called "5V"
or "MISO" could help a lot in understanding the circuit. Many short
nets don't need a name or clarification, and adding names would hurt
more due to clutter than they would illuminate. Again, the whole point
is clarity. Show a meaningful net name when it helps in understanding
the circuit, and don't when it would be more distracting than useful.
Keep names reasonably short
Just because your software lets you enter 32 or 64 character net
names, doesn't mean you should. Again, the point is clarity. No
names is no information, but lots of long names are clutter, which then
decreases clarity. Somewhere in between is a good tradeoff. Don't get
silly and write "8 MHz clock to my PIC", when simply "CLOCK", "CLK", or
"8MHZ" would convey the same information.
See this white paper for recommended pin name abbreviations. Compiled by Altium based on ANSI/IEEE standards.
Upper case symbol names
Use all caps for net names and pin names. Pin names are almost
always shown upper case in datasheets and schematics. Various schematic
programs, Eagle included, don't even allow for lower case names.
One advantage of this, which is also helped when the names aren't too
long, is that they stick out in the regular text. If you do write real
comments in the schematic, always write them in mixed case but make sure
to upper case symbol names to make it clear they are symbol names and
not part of your narrative. For example, "The input signal TEST1
goes high to turn on Q1, which resets the processor by driving MCLR
low.". In this case, it is obvious that TEST1, Q1, and MCLR refer
to names in the schematic and aren't part of the words you are using in
Show decoupling caps by the part
Decoupling caps must be physically close to the part they are
decoupling due to their purpose and basic physics. Show them that way.
Sometimes we see schematics with a bunch of decoupling caps off in
a corner. Of course, these can be placed anywhere in the layout, but by
placing them by their IC you at least show the intent of each
cap. This makes it much easier to see that proper decoupling was at
least thought about, more likely a mistake is caught in a design review,
and more likely the cap actually ends up where intended when the layout
Dots connect, crosses don't
Draw a dot at every junction. That's the convention. Don't be lazy.
Any competent software will enforce this any way, but surprisingly we
still see schematics without junction dots here occasionally. It's a
rule. We don't care whether you think it's silly or not. That's how
Sort of related, try to keep junctions to Ts, not 4-way crosses.
This isn't as hard a rule, but stuff happens. With two lines crossing,
one vertical the other horizontal, the only way to know whether they are
connected is whether the little junction dot is present. In past days
when schematics were routinely photocopied or otherwise optically
reproduced, junction dots could disappear after a few generations, or
could sometimes even appear at crosses when they weren't there
originally. This is less important now that schematics are generally in
a computer, but it's not a bad idea to be extra careful. The way to do
that is to never have a 4-way junction.
If two lines cross, then they are never connected, even if after some
reproduction or compression artifacts it looks like there maybe is a dot
there. Ideally connections or crossovers would be unambiguous without
junction dots, but in reality, you want as little chance of
misunderstanding as possible. Make all junctions Ts with dots, and all
crossing lines are therefore different nets without dots.
Look back and you can see the point of all these rules is to make it as
easy as possible for someone else to understand the circuit from the
schematic, and to maximize the chance that understanding is correct.
There is a human point to this too. A sloppy schematic shows lack of
attention to detail and is irritating and insulting to anyone you ask to
look at it. Think about it. It says to others "Your aggravation with
this schematic isn't worth my time to clean it up" which is basically
saying "I'm more important than you are". That's not a smart thing
to say in many cases, like when you are asking for free help here, handing
in homework, or showing your schematic to a customer.
When you give someone a sloppy schematic (or any other sloppy work from
you), the first thing they're going to think is "What an asshole".
Everything else they think of you and your work will be colored by that
first impression. Don't be that loser.