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.)
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.”
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.
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.
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).
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.
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.
S1 is an
SPST switch
that turns
the power to the circuit on or off.
An
electret microphone
(MIC) transforms
sound waves into electrical signals.
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.
C1
is a
capacitor
connected between
R1 and R2.
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.
IC1 is an
audio amplifier (op
amp)
connected at Pin 3
to R2.
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 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.