
How to Read a Schematic - jlturner
https://learn.sparkfun.com/tutorials/how-to-read-a-schematic
======
rramadass
I am actually quite disappointed with sites/books which talk about circuit
"schematics". They are all too basic. What i am looking for is a book which a)
Will take a non-trivial schematic for an actual product and walk me through
deciphering it b) Will show me a actual PCB and derive its schematic from it.
This is kind of like trying to understand the architecture of a system given
its codebase and documentation. While there are lots of books which walk you
through understanding software (eg. Linux Kernel) i have not found one which
will do the same for electronic schematics. As a person self studying
Embedded/Electronics systems, i find this a big drawback for real-world
understanding. Perhaps some experts here can chime in with suggestions on how
i can acquire such a skill.

~~~
Animats
"The Art of Electronics", by Horowitz and Hill, is a respected textbook for
this. The ARRL Handbook has many explained schematics for radio.

For modern commercial products, mostly you'll have some big special purpose
ICs plus some minor components for power and noise management. The schematic
won't tell you much because all the action is inside the ICs.

Here's something of mine you can look at, a design on Github made with
KiCAD.[1] The schematic is here.[2] All the files to make a board are there,
and both I and others have had working boards fabbed from those files.

The application is unusual - it's an interface for antique Teletype machines
that need signals of 60mA at 120V. There are no off the shelf ICs for that. So
there's a custom switching power supply to make that voltage from a 5V USB
port. The README file for the project explains how it all works. It has all
the extra parts you need in the real world to handle USB hot-plugging, keep
the switcher noise out of the USB connection, keep RF noise down, and protect
the circuit against a shorted output or a big inductive kick-back from the
load.

The data sheet for the LT3750, the controller for the switching power supply,
is essential when reading the schematic.[3]

You can download KiCAD and play with the files. You can also download LTSpice
and run a simulation; the files for that are in the repository.

This is complex enough to be non-trivial, yet simple enough to be
understandable.

[1] [https://github.com/John-Nagle/ttyloopdriver](https://github.com/John-
Nagle/ttyloopdriver) [2] [https://raw.githubusercontent.com/John-
Nagle/ttyloopdriver/m...](https://raw.githubusercontent.com/John-
Nagle/ttyloopdriver/master/board/images/schematic.png) [3]
[https://www.analog.com/media/en/technical-
documentation/data...](https://www.analog.com/media/en/technical-
documentation/data-sheets/3750fa.pdf)

~~~
brianpgordon
I'm tinkering with electronics and am finding The Art of Electronics very
good. At the same time, I'm going through "Practical Electronics for
Inventors" \- for non-advanced topics it basically covers the same information
as The Art of Electronics but in more of an informal/intuitive style than even
the decidedly informal Art.

~~~
rramadass
I found "Designing Embedded Hardware by John Catsoulis" quite good in addition
to the above two.

------
metaphor
In all seriousness, the absolute crap that manifests from schematics developed
in Eagle coming out of the open source community is a personal pet peeve. An
example of almost everything you shouldn't be doing with respect to at least
the following can be found here: color, symbology, scale, reference
designators, typeface, labeling, orientation, functional grouping, density,
overlap, flow, etc. It's a shame there's zero reference to industry standards
like IEEE Std 315 and ASME Y14.44.

Abuse of color is probably the one thing that really drives me nuts. I'm of
the opinion that if electrical intent cannot be cleanly and unambiguously
conveyed in black and white, it's garbage.

I secretly wish EEs in general exercised the same level of meticulous
attention to detail and pride that designers in the typeface community do, in
the same vein as this comment[1].

[1]
[https://news.ycombinator.com/item?id=19608359](https://news.ycombinator.com/item?id=19608359)

~~~
ultrarunner
Haven't ever popped open anything that came in an Amazon box? At least with
OSHW _you_ can submit corrections to bring into compliance with any standard
you find relevant. Better than firing off a sternly written letter to China
(or HN).

~~~
metaphor
The OSHW community has every right to distribute crap schematics, but it isn't
immune to criticism.

For the record, I haven't opened an Amazon box of my own purchase since 2016
(dropped that like a bad habit after 10 years of Prime patronage), although
I'm admittedly failing to see what the relevance of Amazon and China have to
do with this discussion. I don't contribute to the OSHW movement because,
quite frankly, if I can't buy the custom hardware solution I need, I'll design
it myself.

------
Taniwha
Some hints on how to get to the meat of a circuit:

The most important thing is to learn what to ignore, stuff that has to be
there to help a circuit work, but that don't tell you much about what it does

\- ignore all the power stuff

\- in particular capacitors oriented with the two lines horizontal are usually
decoupling capacitors, ones oriented the other way tend to carry interesting
signals

\- learn to find and ignore the biasing resistors around transistors

Rules of thumb:

\- outputs of transistors are usually inverted from their inputs if taken from
the collector/drain

\- signals usually move from left to right

\- chips vaguely have inputs on the left and outputs on the right (or pins
that work together grouped together)

~~~
Taniwha
Forgot one of the most obvious:

\- Positive voltages tend to be above negative voltages in a schematic (ground
is usually more negative than the power rail) nodes between them on the page
tend to be between those voltages

------
gnode
I have more experience in reading analogue schematics, and I imagine this is
less challenging in purely digital electronics, but to me understanding a
schematic is really more about identifying the causal flow of information /
signals, by recognising familiar patterns (amplifiers, filters, resonators,
etc). This article is more an alphabet chart.

------
Ace17
Too bad, the article only deals with how to recognize individual components
(like a french dictionnary that would be entitled "how to read french").

------
fake-name
One thing to be aware of: spark fun's schematics are generally flaming crap,
along with most arduino stuff.

> Sometimes, to make schematics more legible, we'll give a net a name and
> label it, rather than routing a wire all over the schematic.

Net labels are the anti-readable-schematic. They're basically the electronics
equivalent of GOTO. This is total bullshit.

People use net-labels when they don't want to bother spending time to make
their schematic flow properly, or they're doing something dumb like drawing
their schematic symbols with the pin-positions from the physical IC.

Good schematics should make the overall operation of the system easy to
discern. You should have a consistent signal-flow (generally left-to-right,
though the opposite also works). More complex systems should also use a
hierarchical design with the interfaces between logical sections being clearly
defined.

~~~
snops
I think you are being quite harsh, net labels are fine for embedded/digital
stuff, which generally consists of quite separate functional blocks with few,
easily reasoned signals between them (e.g. power + I2C). A lot of Sparkfun's
products fall into this category.

The Arduino Uno by contrast uses routing too much in my opinion, and is quite
hideous: [https://www.arduino.cc/en/uploads/Main/arduino-uno-
schematic...](https://www.arduino.cc/en/uploads/Main/arduino-uno-
schematic.pdf)

If you look at say laptop or SoC development board schematics, they are full
of net names, and they certainly aren't using physical pin positions. In fact
quite the opposite, one physical device is often split into multiple schematic
symbols (e.g. supply rails are one symbol). For the overall system, they use a
block diagram or hierarchical schematic. Here is the BeagleBone schematic,
which I think makes good use of net labels, e.g. the BAT connection on page 2:
[https://beagleboard.org/static/beaglebone/BEAGLEBONE_SCHEM_A...](https://beagleboard.org/static/beaglebone/BEAGLEBONE_SCHEM_A3.pdf)

I will agree with you on net names for analog systems, such as audio
amplifiers or complex power supplies. There, net names can quite often be
misused, as you need to keep more connections in your head when trying to
understand a circuit, and it is much harder to remember all the net names than
see a direct connection.

~~~
fake-name
As I said, net labels are like GOTO. There are very specific situations where
GOTO (or net labels) are useful.

They're also used in lots and LOTS of other situations where they're actively
harmful.

Contexts like spark-fun designs, where the layout is almost always _only one
page_ is a situation where they're actively harmful.

\------

I've done large, complex system designs without using net-labels at all. It
takes a lot of thought about how to structure your component symbols, and was
considerably helped by my EDA tool (Altium) having a facility to bundle
multiple logically associated but non-bus wires into a single logical
connection ("signal harnesses" \- they're _great_ ), as well as very robust
hierarchical design facilities.

It's absolutely possible to do, and you wind up with a much nicer schematic in
the end, but it does take longer.

On the other hand, a site like spark-fun in particular sells _educational
components_. The fact that they _don 't_ have decent schematics is really
extremely disappointing.

------
mbell
A question: How many people are there out there that understand what these
devices are whom are in need of a(nother) symbol reference?

~~~
SlowRobotAhead
I’m always having to double check where arrow (body diode) is on a Pch vs Nch
mosfet... so... at least one.

I think the market of people just learning that would like a nice reference
like this is probably larger than the number of people who would complain
about educational resources being availble. :)

~~~
userbinator
_where arrow (body diode) is on a Pch vs Nch mosfet..._

P points to N. This general rule applies to the symbols for diodes, BJTs,
UJTs, JFETs, and other semiconductor components too.

------
cr0sh
Probably already mentioned, but looking at the "schematic symbols" \- I
noticed they didn't mention the (usually international) form of values - ie
2k5 - meaning 2.5k.

Also - they show symbols for potentiometer and variable resistor, but they
don't show how if you connect the wiper to one end of the potentiometer, that
means "variable resistor" (and is how you usually make one - unless using the
potentiometer as an actual rheostat, which means connecting between one end
and the wiper, leaving the other end free; electrically it's equivalent to not
leaving the wiper floating with respect to the other end, but in a real
rheostat, there is no "other end" to connect with - not without modification
of the rheostat).

Then the battery symbols - and this isn't on them, but more the "standard";
yes, positive is the long side for a series of cells - but on virtually every
standard consumer battery cell (C, D, AA, AAA), which end is the positive end?
Yep - the one with the "bump" or "short end".

Even on the 9 volt battery, the smaller terminal is the positive end.

The only time things are "proper" are with coin cells, where the positive end
is the flat plate (usually with writing), with the negative being the smaller
plate surrounded by the edge of the positive plate on the other side. Knowing
humans, though, I wouldn't be surprised if there was a coin-style cell where
that was reversed...

I expect, though I don't know, that the battery stuff is the result of history
(much like how positive and negative aren't really correct per physics as to
how current really "flows").

I'm sure there's more weird stuff in Sparkfun's tutorial; it seems mainly
geared for an American audience, and also toward older-styled symbols (like
you'd find on schematics from the 1960s-80s). Most of the newer style haven't
changed too dramatically - but there are some differences (they did note the
resistor differences - but I've seen some others).

I'll have to read this in more detail later...

~~~
iheartpotatoes
'.' and ',' generally can be rubbed off or overlooked if the font is too small
or if printed on paper/plastic, etc, hence the more reliable notation you
mentioned.

------
JoachimS
The text shows US and international symbols for things like resistors. But
when it comes to logic gates don't mention the IEC standard gate symbols. Like
these:

[https://commons.wikimedia.org/wiki/Category:IEC_Logic_Gates](https://commons.wikimedia.org/wiki/Category:IEC_Logic_Gates)

~~~
Doxin
I'm fairly sure no one uses those, I've never seen them in the wild anyways,
and I live in a country where they use the international symbols otherwise.

~~~
GlennS
I love this use of "in the wild".

The electronics jungle. Intreprid explorers peering through the gaps between
the cables.

------
g3ph4z
Oh wow, I just found it few weeks ago. It was really helpful for me.

------
spalt
"I understand how to read a schematic!" \- John Hammond

