Constructing Your Circuit on a Printed Circuit Board (PCB)
When you’re satisfied with the operation of your circuit, the next step is to build a permanent version of the circuit. Although there are several ways to do that, the most common is to construct the circuit on a printed circuit board, also called a PCB. In the following sections, you learn how printed circuit boards work and how to assemble the coin-toss circuit on a PCB.
Understanding how printed circuit boards work
A printed circuit board is made from a layer of insulating material such as plastic or some similar material. Copper circuit paths are bonded to one side of the board. The circuit paths consist of traces, which are like the wires that connect components, and pads, which are small circles of copper that the component leads can be soldered to. Figure 6-13 shows a typical printed circuit board.
FIGURE 6-13: A printed circuit board.
There are two basic styles of printed circuit boards available:
Through-hole: A PCB in which the copper circuits are on one side of the board and the components are installed on the opposite side. In a through-hole PCB, small holes (usually inch in diameter) are drilled through the board at the center of the copper pads. Components are mounted to the blank side of the board by passing their leads through the holes and soldering the leads to the copper pads on the other side of the board. When the solder joint is completed, any excess wire lead is trimmed away.
Surface-mount: A PCB in which the components are installed on the same side of the board as the copper circuits. No holes are drilled.
Surface-mount PCBs are easier for large-scale automated circuit assembly. However, they’re much more difficult to work with as a hobbyist because the components tend to be smaller and the leads are closer together. Thus, all the PCBs used in this book are of the through-hole variety.
Using a preprinted PCB
The easiest way to work with printed circuit boards is to purchase a preprinted board from an electronics parts supplier. You’ll find a wide variety of preprinted PCBs at online distributors such as Hobby Engineering (www.hobbyengineering.com), Jameco Electronics (www.jameco.com), or All Electronics (www.allelectronics.com).
As you can see, preprinted PCBs come in a wide variety of shapes and sizes. The most useful, from a hobbyist’s point of view, are the ones that mimic the terminal-strip and bus-strip layout of a solderless breadboard. For example, Figure 6-14 shows a PCB that has 550 holes laid out in a standard breadboard arrangement. With a preprinted PCB that has a breadboard layout, you can transfer your breadboard prototype circuit to the PCB without having to come up with an entirely new layout.
FIGURE 6-14: A preprinted PCB that uses a standard breadboard layout.
Some preprinted circuit boards have layouts that are similar to standard breadboard layouts, but not identical. So check carefully before you build; you may have to make minor adjustments to your circuit layout to accommodate the PCB you’re using.
CREATING A CUSTOM PCB
As you get more advanced in your electronics skills, you may find that you want to create your own custom-printed circuit boards rather than force your circuit designs to fit the limited variety of preprinted circuit boards that are available. Although it isn’t a trivial or inexpensive process, it’s possible to make your own printed circuit boards customized for your circuit.
Here are the basic steps for creating your own printed circuit boards:
1 Purchase a blank PCB. The entire surface of one side of this board will be completely coated with copper.
2 Create a mask on the copper surface that indicates the circuit layout.There are several ways to do this. For simple circuits, you can simply hand-draw the circuit onto the copper using a special pen designed for this purpose. For more complicated circuits, you can purchase special stickers that are shaped like pads and common traces and place the stickers directly on the copper. Or you can design the circuit on your computer using any graphics drawing software, print the design on special paper, and transfer the design to the copper using a hot iron.
3 Etch the board by dipping it into a special chemical that eats away all the copper that isn’t covered by your mask.This is a nasty process that needs to be done outside in a well-ventilated area while wearing gloves, a face mask, and goggles. When the etching is finished, all the copper that wasn’t covered by the mask will be gone.
4 Wash the board to get all that nasty copper etching solution off.
5 Scrub away the mask to reveal the beautiful copper circuit pattern left behind.
6 Drill holes in the center of each pad, and then assemble your circuit.
Note that there are several companies on the Internet that will make small printed circuit boards for you. It isn’t cheap, but the price per board comes down dramatically if you have them make more than just one. For example, PCBWay (www.pcbway.com) will make 4-inch-square circuit boards for about $3 per board if you order ten.
Building the coin-toss circuit on a PCB
This section presents a complete procedure for building the coin-toss circuit on a small preprinted PCB. When you get all your materials together, you should be able to complete this project in about an hour.
All the parts required to build this prototype circuit can be purchased from your local RadioShack. Or, you can order them online from any electronic parts supplier. For your convenience, here is a complete list of the parts you’ll need to build this prototype circuit, along with the RadioShack catalog part numbers:
| Quantity | Description |
|---|---|
| 1 | General-purpose, dual-printed circuit board |
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