Drawing Schematic symbol using eagle

Tutorial 3: Drawing Schematic symbol

1: Use component designators

This is so much automatic with any of schematic capture programs, but we still often see schematics without them. If you are going to draw your schematic on a napkin and then scan it, make sure to add component designators. These intend to make the circuit very easier and quite professional to talk about. I have skipped over questions when schematics did not have component designators for I did not feel like troubling with the second 10 kΩ resistor from the left by the top push-button. It’s a lot easier to say R1, R5, Q7, etc.

2Clean up text placement

Part names and values are generally plunk down by schematic programs, based on a generic part definition. This means they often end up in unsuitable places in the schematic when other parts are lain nearby. Fix it!. That is the part of the job of drawing a schematic. Some schematic capture softwares make this effortless and easier than others. In Eagle as an example, unfortunately there can only be one symbol for a part. Some parts are usually placed in different alignments and positions, horizontal and vertical. In the resistor’s case, for example, diodes can be placed in at least 4 alignments and positions since they have directions too. The placement of text around a part, like the component designator and value, probably won’t work in other alignments and positions than it was originally drawn in. Well, if you rotate a stock part, move the desired text around afterwards so that it is readable, clearly belongs to that part, and does  not collide with other parts of the drawing. Vertical text looks stupid and makes the schematic hard to read.

I create different unnecessary parts in Eagle that differ in the symbol orientation and therefore the text placement. That is much work up front, but does make it easier when drawing a schematic. However, it does not matter how you achieve a clear and neat end result, only that you do. There is no excuse. Sometimes we hear whimpers like “But CircuitBarf 0.1 does not let me do that”. So get something that does. Besides, CircuitBarf 0.1 probably does let you do it, just that you were much lazy to read the guide to learn how, and so gloppy to care. Draw it (neatly!) on paper and scan it if you have to.

schmatic symbol 125

Let’s take an example, here are some parts at different orientations. Note! how the text is in different places respective to parts to make things clear and neat.

3: Basic layout and flow

In general it is quite better to put high value voltages towards the top, lower value voltages towards the bottom, and logical flow from left to right. That’s clearly not possible all the time, but at least a general higher level effort to do this ‘ll greatly illuminate the circuit to those reading your schematic.

One most noteable exception to this are feedback signals. By their very nature, they feed “back” from downstream to upstream, so they have to be shown sending information opposite to the main flow.

Power connections have to go up to positive voltages and down to negative voltages. There was not 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 sub-circuits to be drawn similarly to most of the time. Once you get more experience observing the schematics, these will pop out at you and you will appreciate it. After some comprehension and perception you realize “Oh, it’s a common emitter amplifier. Why did not that *** just draw it like one in the first place!? :

 

4: Draw pins according to function

Must show pins of ICs in position that is relevant to their function, NOT HOW THEY HAPPEN TO STICK OUT OF THE CHIP. Must try to put positive power pins at top, negative power pins (usually grounds) at bottom, inputs at left, and outputs at right. Must note that this suits the general schematic layout as described above. Of course this is not always reasonable and possible. General purpose parts like micro-controllers and FPGAs have pins that can be input and output depending on use and can even then vary at the run time. You can put at least 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 quite difficult to understand. Some people use the excuse that this aids in debugging, but with a littleidea you can see that it is not true. When you want to look at something with a scope, which question is much 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 unusual cases you might want to go around and fetch about an IC and look at all the pins, but the first question is by far more common.Physical pin order layouts obscure and unclear the circuit and make debugging more difficult. Do not do it

5: Direct connections, within the 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 is not always possible or reasonable. Obviously it cannot be doneby means of multiple number of sheets, and also a messy rat’s nest of wires is worse than a few “air wires” that are carefully chosen. It is impossible to devise a universal rule here, but if you continuously think of the fabulous 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 perceive and understand the circuit easily and simply, not make them figure it out despite the schematic.

6: Design for regular size paper

The time of electrical engineers having drafting tables and being set up to work with D size drawings is 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 is no room on the desk, it ends up covering the keyboard, pushes things off your desk when you move it, etc.

screen at about the actual and same size. Currently, the largest common screen size. The point is design your schematic so that individual sheets are quite readable on a single normal page. Having to scroll a page at that resolution to look at important detail is annoying. If that means using more pages, go ahead. You can flip pages back and forth with a single button press while using the Acrobat Reader. Flipping pages is preferable to panning a large drawing or dealing with exceptionally large paper. I also find that one normal page at equitable detail is a good size to show a subcircuit.Alsothink about pages in the schematic like paragraphs in a narrative.However, breaking a schematic into individually tagged or labeled sections by pages can actually help enhance the readability if done right. For example, you might have to grab a page for the power input section, the immediate micro-controller connections, the analog inputs, the ethernet interface,the H bridge drive power outputs etc. It is actually fruitful to break-up the schematic in this way even if it had nothing to do with the drawing size.

7: 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 that 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 demoninator or 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 whatsoever. Sometimes these even give you the sheet and coordinates of one or more corresponding jump points. That is 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 does not 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 type 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 exact 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 you 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 & do not when it would be more distracting than useful.

8: 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 ANSI/IEEE standard for recommended pin name abbreviations.

9: Upper case symbol names

Use all caps for net names and pin names. Pin names are almost always shown upper case in schematics and datasheets. Various schematic programs, Eagle included, don’t even allow for lower case names. One benefit of this, which is also helpful when the names are not too long, is that they stick out in regular text. If you want to write real comments in the schematic, always write them in mixed case but make sure to upper case symbol names to make it clear that they are symbol names, not part of your narrative. For example, “The input signal TEST1 goes high to turn on Q1, which then resets the processor by running MCLR low.”. In this case it is obvious that TEST1, Q1, and MCLR refer to names in the schematic and are not part of the words you are using in the description.

10: Show decoupling caps by the part

Decoupling caps should by physically close to the part they are decoupling due to their purpose and basic physics. Show’em that way. Sometimes I have seen schematics with a bunch of decoupling caps off in a corner. Of course these may 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 pretty easier to see that proper decoupling was at least thought about more likely a mistake is caught in a design review & more likely the cap actually ends up where intended when layout is done.

11: Dots connect, crosses don’t

You have to draw a dot at every junction. That’s the convention. Don’t be lazy. Any competent software will enforce this anyway, but surprisingly we still see schematics without junction dots here occasionally. It is a rule. We do not care whether you think it is silly or not. That is how it is done.

Sort of related, try to keep junctions to Ts not 4-way crosses. This is not as hard as a rule but stuff happens. With two lines crossing, one is vertical the other is horizontal, the only way to know whether they are connected or not is the presence of little junction dot. In past days when schematics were regularly photocopied or otherwise optically reproduced, junction dots could disappear after a few generations or could sometimes even appear at crosses when they were not there originally. This is less important now that schematics are generally in a computer, but it is 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, they are never connected even if after some reproduction or compression artifacts. It looks like there maybe a dot. Ideally, connections or crossovers would be clear and 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.

Good schematics show you the circuit. Bad schematics make you decipher them.

There is another human point to this too. An unsystematic schematic shows lack of attention to detail and is a irritation and insult to anyone you ask to look at it. It says to others “Your aggrevation with this schematic is not worth my time to clean it up” which is basically saying “I am more important than you are”. That is not a smart thing to say in many cases like when you are asking for free help here, showing your schematic to a customer or teacher  etc.

Read>> Plumbing pipe and fitting symbols Read>> Hydraulic And Pneumatic Schematic Symbols

 

Neatness and presentation count a lot…!

You are judged by your presentation quality every time you present something, whether you think that is how it should be or not. In most cases people will not bother to tell you either. They will just go on to answer a different question & won’t look for some good points that might make the grade one notch higher or hire someone else etc. When you give someone a sloppy schematic (or any other sloppy work from you), the first thing they are going to think is“What a jerk”. Everything else they think of you and your work will be colored by that initial impression.

Watch this video & learn how to draw a circuit diagram!

Tutorial 1: Pre-requisites to draw schematic diagram
Tutorial 2: Components connectivity and polarity
Tutorial 3: Drawing Schematic symbol
Tutorial 4: Optimizing your circuit schematic diagram

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