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Schematic capture is the beginning of the process whereby a block diagram is converted into a pcb. Generally, schematic capture programs use a library which associates a graphical symbol with one or more part numbers (e.g. 1 kohm resistors of a certain form factor, but from different manufacturers); symbols from this library are placed on schematic sheets, and the schematic capture program provides a method for wiring these symbols together to create a functional circuit. The database is then passed to a layout program for parts placement and trace routing and finally a pcb artwork file is generated.

Common schematic capture programs include Protel, ViewLogic, and Mentor Graphics.

So you want to build a robot, or perhaps an effects pedal for your guitar, or a strobe light. Maybe your job requires you to design circuits, or capture the designs of other engineers. It is likely you will run into a situation where schematic capture is necessary. Many hobby kits come with premade printed circuit boards, but for those with the right software, the right equipment, and a sense of adventure, there is the opportunity to see your very own circuit through from idea to actualization.

There are numerous schematic capture programs out there, which vary greatly in size and capability. Some are small, inexpensive (or free!) and suitable for electronics hobbyists. Others are huge and very expensive, and generally only used by businesses. If you want a free schematic capture program, your best bet is to do a Web search and pick the program that best suits your needs. Some are limited-time demo programs; I tend to avoid these, because you never know how long a project may take. Plus, many of the time-limited demos are in fact very expensive if you actually want to buy them. Most of the free time-unlimited schematic capture programs have some sort of limitation, such as maximum PCB size or maximum number of components allowed on your diagram. These limits are generally quite easy to work around, and are not likely to affect you if you are making a small, fairly simple circuit.

Though there are many differences between the various schematic capture programs out there, there are plenty of common characteristics that seem to appear in most every program. There are also some general guidelines to follow when creating a schematic that are independent of the software used, and will help you create a more organized design.

You are likely to encounter the following in any schematic capture program:

  • Sheets - Individual pages in a schematic. Very simple designs might only fill a single sheet; I have seen circuits that require over 20 sheets.
  • Component libraries - Pallettes or drop-down menus that allow access to a variety of circuit componets, such as resistors, capacitors, and integrated circuits.
  • Wires - Schematic capture programs allow you to draw lines between components that represent the actual, physical connections between parts.

When you connect components in a schematic, you are actually creating a database that stores the relationships between all the parts in your design. This database is called a netlist, and is often viewable as a text file. It is very important to check this netlist when you think your design is complete; often, surprises are found here that require you to go back and edit the schematic.

Use common sense when placing your components on a sheet. Do not be afraid to use multiple sheets; overcrowding makes a schematic difficult to read and difficult to edit if it turns out you need to make changes later on. If you have a large IC, it is possible that that IC might be the only component on a sheet. Where I work, we use offpage connectors extensively; these allow for a very neat look and a more organized design, especially when using a lot of digital components.

There are some very, very simple schematic programs out there that are ONLY for schematic drawings: that is, the only result of your drawing is a pictorial representation of your circuit. However, if you actually plan to build something, it is likely you will be using software that stores a lot of information in the background for later use. For instance, components you pull from a library often have a part number and a package type. The part number identifies the part as a specific component made by a particular manufacturer. The package type can indicate the size, shape, assembly type (surface mount or through hole), and number of pins. It is very important to pay attention to the package type, especially if board area is a concern.

If you are building something in your own home for fun, it is likely that you will be using mainly through-hole parts. Surface mount parts are often very tiny and difficult, if not impossible, to solder by hand. It is also likely that you will be using relatively low pin-count parts; you probably won't need a 300-pin FPGA for a hobby project. Therefore, your schematics are likely to be fairly simple. Just make sure you choose carefully from the part library, and know the exact part number of the part you want to use.

Most schematic software allows you to enter text labels on the schematic sheet, and possibly to draw shapes and lines that will not become part of the component database. It is best to label the functional blocks of your circuit; this makes it easy to quickly identify the parts of your circuit. This is especially important if you are working with other people who will need to use your schematic for reference.

Like any skill, schematic capture becomes easier with practice. This is especially true for routing the wires from one component to another; my first few schematics were an awful mess! So if you are thinking of building your own electronic invention, consider taking the time to learn to use some schematic capture software. Though it can be tedious at times, it it ultimately quite rewarding.

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