Electronics_Projects_For_Dummies

60 Part I: Project Prep

Schematics help you understand how a particular electronics project works as well as how to build it. You can build the circuit on a breadboard (more on that in the upcoming section, “Breadboarding”) by inserting the components and making the connections on the board that are indicated by the schematic.

Perusing a simple schematic

An example of a very simple schematic shows a battery, one electronic component, and the wires connecting them. Figure 4-1 shows a schematic that contains a 1.5 volt battery, a wire from the positive side of the battery (+V) connecting it to one of the leads on an LED, and a wire connecting the other lead of the LED to the negative side of the battery. With both wires connected, current flows from one terminal of the battery through the LED, making it light up, and back to the other terminal of the battery. (If the LED is connected to only one battery terminal, no current flows, and the LED will not light up.)

Figure 4-1:

A simple circuit with a battery.

Some circuits use too many components for the schematic to show the wire connecting every component to the battery. In those cases, we use a convenient symbol for a voltage source to represent the positive side of the battery and a ground symbol to represent the negative side of the battery, as shown in Figure 4-2. This is same circuit as the preceding figure with a voltage source symbol and ground symbol representing connections to the battery. These symbols are also used in applications where a metal chassis is used as ground and a power supply is used to supply voltage.

You can read more about connecting to +V and ground in the later section, “The anatomy of a breadboard.”

Chapter 4: Running Down the Skills You Need 61

1.5V

Figure 4-2:

This circuit shows a voltage

source

symbol and LED1 ground

symbol representing connections

to the battery.

Interconnections among components (how wires are connected to move electricity from one component to another) in a circuit are made with wires or bits of copper placed on a breadboard. A schematic won’t usually specify which kind of connection you are using, only that a connection exists. Figure 4-3 shows a few methods of representing interconnections.

Figure 4-3:

How connections and nonconnections are represented in schematics.

UNKNOWN

62 Part I: Project Prep

You’ll also find symbols for commonly used components, such as resistors, diodes, capacitors, and transistors, in schematics (see Figure 4-4). Chapter 3 explains what most of these components do in a circuit.

Figure 4-4:

Common component symbols.

Switching gears with switches

You use switches to turn power to a circuit on or off or to connect/disconnect a pin on a component to +V or ground. However, switches don’t use just a single symbol. Rather, they come in several varieties, indicating

How many wires they control

Whether they stay in the position you set them at or return to a normal position after you release them

An SPST (single-pole, single-throw) switch has one incoming wire and one outgoing wire that you use to open or close a connection in a circuit. For example, if a wire runs from the negative pole of a battery to an SPST switch and another wire runs from the SPST switch to a circuit, current can flow through the circuit when you have the switch in the closed position. When you flip the switch to the open position, no current can flow through the circuit.

SPST switches also come in the momentary switch variety; these can be normally open (NO) or normally closed (NC) and are generally controlled by pushbuttons or relays. A normally open switch conducts current only when the button is pressed and returns to its open position when it’s released. A normally closed switch won’t conduct current when you press the button but returns to its normal position and conducts current when you release it.

An SPDT (single-pole double-throw) switch has one incoming wire and two outgoing wires that you use to control which of two components is connected. Suppose that the incoming wire is connected to power, one outgoing wire is connected to a green LED, and the other outgoing wire is connected to a red LED. When you have the switch in one position, the green LED lights up; when you flip the switch, the green LED goes dark, and the red LED lights up.

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You can think of a DPDT (double-pole double-throw) switch as containing two SPDT switches that switch in tandem. To see an example of this in action, visit Chapter 13, where we use DPDT relays to simplify the wiring of our breadboard.

Schematic variables

Some components are polarized, which means that you have to put them in the circuit in a particular way. Schematics can identify the polarity of components (see Figure 4-5).

Relay

64 Part I: Project Prep

Identifying the + lead on polarized capacitors and LEDs is easy because the + lead is longer than the ground lead. For transistors and integrated circuits (ICs), you have to take a look at the datasheet to find out which pin to connect to +V and which pin to connect to ground. A datasheet is the manufacturer’s specifications for the component. You can read about pins in Chapter 3.

Some components are variable, meaning that they don’t just operate at one value; instead, you can adjust their values. Variable resistors (also called potentiometers), variable capacitors, and variable coils are all examples of adjustable components. You can use these adjustable items to control volume or tune in a radio station, for example.

Pulling it all together

After you understand some of the elements that go into a schematic, we thought you’d like to have us rundown how to read a simple schematic sample. The schematic used in Chapter 6 is shown in Figure 4-6. This is a circuit that includes a microphone and an IC that amplify noises; the circuit works together with a parabolic (curved) metal dish that helps pick up sounds.

parabolic microphone.

First, some general rules: A line between two symbols indicates that the two components are connected by a wire. A connection is also indicated when a symbol is shown connected by one line to another line with a dot at the junction.

68 Part I: Project Prep

Neatness counts: Take your time to make your board neat and tidy. This helps you to avoid mistakes and also helps you to troubleshoot if things aren’t working quite right.

Spacing: Leave yourself room to place items, allowing a little space between them. It’s better to leave a little more space between elements and use a bigger or expanded breadboard than to crowd yourself too much. This gives you the space to modify and refine your circuit.

Jumps: Minimize the jumps that you make between connections. (Typically, this involves using a jumper wire.) For example, if you can insert one lead of a component in the same row as the lead of another component you’re connecting to, you don’t have to use a jumper wire to connect them. The less wiring you have, the less messy things get.

Using color-coded wiring helps you to keep track of your layout. For example, many people use black wire for ground and red wire for power. Put wires at 90° angles, not on the diagonal, because diagonal wires will get in the way of other components to be placed on the board.

Also keep wires to a practical length: that is, long enough so you can route them around ICs but short enough so you don’t have lots of extra wire cluttering up your breadboard. (Routing wires around ICs means that if you have to remove or replace an IC, you don’t have to remove all the wires as well!)

Assorted lengths of prestripped wires are available that save you time in cutting, stripping, and bending wires to length. You just pick one that’s already cut to the right length. However, each length of these wires is a different color; thus, if you use prestripped wires, you can’t color-code your circuit. Because most of the photos in this book are black and white, we went for the convenience of using prestripped wires rather than color-coding the wires on the breadboard.

Inserting wires and components

In a nutshell, here’s how to wire a breadboard:

1.Use 22 gauge solid wire to make connections (see Figure 4-9).

Don’t use stranded wire because it can get smushed when you push it into a hole and could even cause shorts in your circuit if a piece of wire breaks off.

See the sidebar, “When stranded wire works,” for times when using stranded wires is more appropriate.