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Drawing Good Circuit Diagrams

Make sure your schematics are not "letters to an unrequited love"

Our work involves designing circuits and assembling devices to customer specification. The creation of circuit diagrams (schematics) is a very important part of this process to which I pay particular attention. Circuit diagrams are important because the quality of the diagram not only has a significant bearing not only on the manufacturing process but also on the efficiency with which modifications can be designed and implemented. In this article, I will talk about two experiences that prompted me to rethink my attitude to schematics.

Don’t make up your own rules.

The first experience involved a wiring modification that was necessitated when a new feature was added to a device. The modification was straightforward, and simply involved adding a few new wires and removing a few existing ones. However, when it came time to implement the modification, there was confusion on the factory floor: adding the new wiring wasn’t a problem, but the team couldn’t work out which wires to remove. When I went down to investigate, I realized that when the device was originally assembled on the basis of a schematic that I had drawn, because, rather than explicitly indicating the physical locations of junctions, the schematic simply denoted junctions with dots, the positioning of junctions had been left to the judgment of the wiring team (see Figure 1).

Figure 1. How the incorrect notation came to be used on the schematic

This meant that when, years later, we attempted to modify the circuit, even the team who performed the original wiring could not say where the junctions were located.
The dot symbol ( ´Żą ) is used chiefly in block diagrams, which show the overall composition of a system, to denote modules connected in series. However, when used in schematics for an actual circuits, dot symbols are unable to express the intended locations of junctions - whether a given junction is supposed to be placed near the input terminals or the output terminals, for instance. To indicate the position of a junction in a circuit diagram, you need to draw a diagonal line originating from the position of the junction (see Figure 2).

Figure 2. The correct way to denote junctions in electronic circuit diagrams

But why had I used dots in my schematic in the first place? Did I copy this notation from somewhere? Had my long working relationship with the wiring team simply caused me to place excess faith in their abilities, and leave them free to wire circuits in the way that was easiest for them? Even I could not remember what my reasons were at the time. I ended up making a lot of extra work for the modifications team, who was forced to remove wires that did not actually need to be removed in order to ascertain where junctions were located.

This experience taught me that failure to observe the established conventions for drawing schematics only creates trouble. After this experience, I became more conscious of the importance of basic tasks, such as identifying the physical positions of the components in order to minimize the length of wiring and considering the amount of separation from other wiring that could potentially be impacted, and now consider these issues myself before issuing instructions. I no longer leave details to the judgment of the wiring team, even if it is a team with whom I have a long working relationship.

"Normal" means different things to different people

The second experience that changed my thinking involved an assembled device that did not conform to the intended design. While I believed that my schematic had been clear, the component connections, wire routings, and wire types used at the time of assembly all differed from what I had intended. I ended up rectifying the situation myself because I didn’t want to wait for the device to be modified. Thanks (?) to that experience, my soldering and crimping skills improved, although I also expended a lot of time and effort and became frustrated, unable to understand why the manufacturing team had not understood my instructions.

Of course, there’s no point brooding on these matters: I decided to go and ask the manufacturing team what had happened. I found out that they had interpreted my instructions differently from how I had intended, that they had different idea of what was "normal", and that things that seemed obvious to me were not obvious to them. That is, both sides had different ideas about how a schematic should normally be interpreted.

You could describe a circuit diagram as a tool that allows the design and manufacturing divisions to achieve a common understanding. However, when you become engrossed in a task, be it drawing schematics or installing wiring (not that becoming engrossed in something is necessarily a bad thing!), it tends to reinforce your (subjective) assumption that ‘you are right’. This means that when a problem does arise, you tend to assume that it is the other party who is at fault. It’s a bit like sending a love letter to an unrequited love: sending a message and having that message understood are not the same thing.

Circuit diagrams should be intuitive

What, then, constitutes a good circuit diagram? By way of an example, let us consider a plastic kitset model. Most commercially available kitsets can be assembled simply by following the instructions, with no need to obtain additional advice. While some hobbyists might build slightly neater models than others, the kitsets allow a consistent result, and the assembler does not have to rely on his or her own judgment about what is "normal". Considering this fact lead me to conclude that a "good diagram" is one that does not require the assembler to rely on his or her own judgment.

It goes without saying that if even the person who drew the schematic can’t work out what it means, you’ve got problems! Even assuming you that do know what you mean, however, you should still aim to draw schematics that can be followed intuitively and that do not require the assembler to use his or her judgment about what is normal or intended. I came to realize that by making circuit diagrams intuitive, I was able to ensure that the devices would be assembled to a consistent standard. Since then, I always put myself in the position of the person interpreting the schematic and ask myself how I can make the schematic clearer and avoid misinterpretation and errors. These days, when the assembled device does not conform to my intended design, I think, "perhaps the schematic was hard to follow". I no longer try and shift the blame to the assembler.

Another thing I am more aware of these days is the importance of making ample opportunities to discuss projects with my team, in order to remove any uncertainty. Doing so has reduced the need to modify products after assembly, thereby helping the production process to run more smoothly.

Obviously, each manufacturer and group will have its own regulations about drawing schematics. When drafting, it is also important to be considerate of your audience, that is, consider how someone working to such rules will interpret your schematic. The same may be said for interpersonal relationships: I believe that better schematics are a way of improving product quality and manufacturing efficiency.

Takahiro Ishikawa
Solutions Development Division, Solutions Development Department

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