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It’s Symbolic: Reading a Schematic

A schematic is your blueprint for building an electronics project. A blueprint for building a house uses various symbols to represent elements, such as doors, and lines to show walls. Instead of doors and walls, the symbols and lines in a schematic represent components such as transistors, integrated circuits (ICs), and resistors as well as the wires that connect them. 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

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.”

figure 4-2

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

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

figure 4-4

Switching gears with switches

You see 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.

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 "Sensitive Sam Walks the Line" project, 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).

figure 4-5

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 "Focusing Sound with a Parabolic Microphone" project 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.

figure 4-6

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.

Here’s what this schematic is saying:

A battery is used to supply 6 volts to the circuit.battery

S1 is an SPST switch that turns the power to the circuit on or off.spst switch

An electret microphone (MIC) transforms sound waves into electrical signals.microphone

A resistor (R1) connects the microphone to the positive battery terminal and supplies the 3 volts required to make the microphone function. Note the dots above and below R1 that indicate connections.resistor

C1 is a capacitor connected between R1 and R2.capacitor

R2 is a potentiometer with one lead connected to C1, one lead connected to the negative battery terminal, and the variable contact connected to Pin 3 of R2.potentimeter

IC1 is an audio amplifier (op amp) connected at Pin 3 to R2.amplifier

Pins 2 and 4 of IC1 are connected to the negative battery terminal.

Pin 6 of IC1 is connected to the positive battery terminal.

Capacitor C2 is connected between Pins 1 and 8 of IC1. The positive side of the capacitor is connected to Pin 1.

Capacitor C3 is connected between Pin 7 of IC1and the negative battery terminal.

Capacitor C4 is connected between Pin 5 of IC1 and the speaker (or headphones).capacitor 4

Capacitor C5 is connected between Pin 5 of IC1 and resistor R3.

Resistor R3 is connected between capacitor C5 and the negative battery terminal.

The speaker (in this case, headphones) is connected between capacitor C4 and the negative battery terminal.