Raspberry Pi Projects For Dummies. Evans Jonathan

to mount buttons onto an enclosure for your project, you’ll probably want to solder wires to the buttons and connect these to your Raspberry Pi. You can even build part of your circuit on a breadboard and use soldered connections for switches or sensors that are located some distance away. (See the upcoming section, “Selecting a soldering iron and accessories,” for more information on what to look for.)

      ✔ A power supply: None of the projects in this book requires a desktop power supply, so this is optional. But for general electronics experimenting, you’ll probably want to have a power supply on hand.

       Selecting a multimeter

      A multimeter is an essential tool for testing, measuring, and diagnosing problems in electronic circuits. You use a multimeter to measure several basic attributes of your circuit, including:

      ✔ Continuity: Whether there is a good connection between two points

      ✔ Voltage: The measure of potential electromotive force in a circuit

      ✔ Current: The measure of the continuous, uniform flow of electrons through an unbroken pathway in an electrical circuit

      ✔ Resistance: Opposition to the flow of current within a circuit

      With a multimeter, you can also measure the voltage provided by batteries and power supplies, and the characteristics of discrete electronic components, such as resistors, capacitors, diodes, and transistors.

       Different models have different features, and the more expensive ones have advanced features you may not need. That said, there are two important features to look for:

      ✔ Continuity with audio signal: Checking continuity – making sure that the things you think are connected really are connected – is the task you’ll use your multimeter for most often. You touch the two probes to part of a circuit to see if they’re connected, and the multimeter screen displays a confirmation. With cheap multimeters, you need to hold the probes in place while looking at the screen, which can be annoying if the probes slip off. It’s a pain to check continuity by holding leads on a circuit while you’re also looking at the display. It’s much easier to just poke around and listen for an audio signal. Meters with audio output will beep when you test for good continuity so you don’t have to take your attention away from the circuit. If you can, spend a little more for a multimeter that has this feature.

      ✔ Auto-ranging: Inexpensive multimeters require you to estimate the range of measurement and set the dial accordingly. On auto-ranging multimeters, you don’t have to set the dial to select the range of measurement that you’re reading. Auto-ranging is particularly handy and can be worth paying slightly more for.

Older multimeters used a needle and graduated scales for the display, but modern ones use a digital readout. If you don’t already have a multimeter, we recommend getting a digital one, like the one shown in Figure 2-2.

       Figure 2-2: A digital multimeter is an essential diagnostic tool.

       Selecting a soldering iron and accessories

      Many of the projects in this book can be built without soldering anything at all, but you’ll need to do a little bit of soldering for some of the projects, so it’s good to have a soldering iron on hand.

      Soldering involves melting solder (a metal alloy that melts at about 700°F) and allowing it to cool, creating a strong, conductive joint. You can solder wires to each other and join wires to components. You can bond wires to circuit prototyping boards such as perfboards and stripboards. Soldering secures components in place, while creating a good electrical connection for a more permanent, longer-lasting project. You can also simply solder certain components (like switches and displays) to wires that lead to your breadboard. That way, you can mount them in a project box. On some projects, you may want to move buttons or switches from the breadboard to the project enclosure, which means you’ll need to solder extension wires on them.

      Your soldering iron provides the heat for creating a soldered joint. Many people have the impression that you melt solder onto the parts that you want to connect, but this is actually backward. When soldering, you use a soldering iron to heat up both the solder and the components that are being joined together. When the components are hot enough, the solder will flow onto them, at which point, you remove the tip of the soldering iron and, thus, the heat supply. The solder cools rapidly and, if done correctly, forms a reliable bond.

Figure 2-3 shows a basic array of soldering tools. The key soldering tools you need at your workbench are as follows:

      ✔ Soldering iron: Your main soldering tool. Irons can be very inexpensive, but the professional ones can set you back hundreds. If you want to save money, avoid the cheapest ones and aim for a soldering iron that’s at the top end of the low-range options. You’ll need one that supplies at least 30 watts. A soldering iron with an adjustable temperature setting can be useful if you need extra heat for large joints, but it’s not essential.

      ✔ Solder: A metal alloy you use to create soldered joints. There are both leaded and lead-free varieties. Some purists prefer leaded 60/40 solder (60 percent tin, 40 percent lead), but lead is toxic, so unless you have a particular need for it, we recommend you opt for the lead-free variety, with a rosin core. The rosin core melts and helps to clean the surfaces you’re joining. Solder comes in a variety of diameters measured in standard wire gauge (SWG). For most electronics soldering needs, 18 SWG or 20 SWG diameter is ideal. You can use 22 SWG for detailed work.

       Somewhat counterintuitively, as the wire gauge number goes higher, the diameter of the wire gets smaller.

      ✔ Extra soldering tips: Tips do the main work of the iron, directing the heat in the right place. Tips come in a variety of shapes and sizes. For most electronics work, you’ll need a cone-shaped tip rather than a chisel tip. Because they come into contact with molten metal and impurities, tips can degrade over time, so it’s a good idea to get spares.

       Different manufacturers have different tip-mounting systems, so buy a couple extra tips when you buy your iron to avoid having to hunt for the right product later.

      ✔ Soldering stand: A device that holds the wand safely while it’s hot. It may have a sponge for cleaning the tip. Soldering stands are often included with soldering iron kits.

      ✔ Cellulose sponge and brass wire sponge: You use these to clean the tip of your iron, which you do while the iron is hot. You can use either a cellulose sponge or a brass wire sponge, depending on your preference. The cellulose sponge can be any garden-variety kitchen sponge from the supermarket dipped in a bit of water and wrung out. Using a moist sponge cools down the tip of the iron, which is something to avoid because your iron will have to work harder to keep the tip at a constant temperature, and contaminants can build up on the tip. The brass wire sponge costs a little more, but it doesn’t cool down the tip of the iron when you’re cleaning it and it doesn’t contaminate the tip. Using brass wire also means that your tip will last longer.

      ✔ Desoldering tools: You use these tools to remove unwanted blobs of solder from your work or disconnect wires, traces, or components that you may have soldered together by accident. You can find both desoldering wick and soldering suckers. A soldering sucker is a spring-loaded pen that you can use to suck liquefied solder away from your work piece. Desoldering wick is simply braided flat copper ribbon, which you press against your work while heating it. Capillary action draws the liquefied solder onto the braid and away from your work. We tend to prefer wick, which is cheaper and usually more effective.

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