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Schematic drawings may show circuitry with separate logic gates or an entire IC package.
A c -B
Here are some common switch types, and some variations you will encounter as you build electronics projects:
A single-pole, single-throw (SPST) switch has one position (on-off) or “throw,” and only one pole.
A double-pole, double-throw (DPDT) switch has two positions (on-on, or on-off-on) and two poles.
Other variations include DPST (double-pole, single-throw) and switches with three or more poles.
In addition to the poles and throws, some switches are spring-loaded (called “momentary switches”). These switches are either normally open (NO) or normally closed (NC). The normal state occurs when the switch isn’t being pressed. For example, a normally open switch doesn’t make electrical contact until you press it.
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136 Part III: Putting It On Paper
Table 6-2 shows several common switch symbols, such as SPST and DPDT, along with a couple of other symbols for such components as speakers, batteries, and incandescent lamps.
Table 6-2 Miscellaneous Component Symbols
Switch, SPDT o—^Z
Switch, DPDT 1
Switch, normally open
Switch, normally closed —Ë I r%—
Piezoelectric buzzer -
Incandescent lamp ^
Getting Component Polarity Right
Many, though not all, components use polarized connections. To make sure that these components function properly, you have to connect them in the circuit in just the right way. In some instances, reversing a component from its proper polarity can permanently damage it and other components in the circuit.
Figure 6-7 shows how schematics identify polarity when using various components.
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Chapter 6: Reading a Schematic 137
Polarity symbols for several components.
NPN TRANSISTOR PNP TRANSISTOR
Be sure to observe polarity when working with the following common components:
Diodes: Including rectifier, Zener, and light-emitting types. Schematics indicate polarity with a short line, which represents the cathode (negative) terminal of the diode.
Some capacitors: Electrolytic, tantalum, and several other special types. Schematics indicate polarity with a + (plus) sign.
Transistors: Schematics show polarity by the type of the symbol.
Logic gates and other integrated circuits: Schematics show polarity by the labels or other markings on the symbol.
Op amps: An op-amp has three connections (besides power): two inputs and the output. The inputs are marked + (non-inverting) and (-) inverting. TERM LinG - LIVE, infDrmatlVE, NDn-CD5t and ÁÅÏÛ1ÏÅ !
138 Part III: Putting It On Paper
Battery: Schematics show polarity with a + (plus) and sometimes also -(negative) sign.
Relay: Coil only. Schematics show polarity with a + (plus) sign.
One Size Fits All: Adjustable Components
Several types of electronic components are adjustable. Instead of operating at just one value, you can manually adjust the component to operate at a range of values.
The most common adjustable components you encounter (whose symbols you can see in Figure 6-8) are
Variable resistor: Also called a potentiometer (or pot). Perhaps the most common of all variable components, you use this resistor for volume control, dimming lights, and thousands of other applications. The potentiometer consists of a resistive element wired between two terminals (such the filament in a lightbulb). On a third terminal, a wiper registers changing resistance as you turn the potentiometer knob.
Figure 6-8 shows how schematics present potentiometers.
^ Variable capacitor: You most often use a variable capacitor in a tuning circuit, such as an AM radio. The capacitor consists of two or more metal plates separated by air. Turning the knob changes the capacitance of the device.
^ Variable coil: Like a variable capacitor, you most often use a variable coil in a tuning circuit. A typical construction uses a coil of wire surrounding a movable metal slug. By moving the slug, you change the inductance of the coil.
Schematic symbols for variable components
use an arrow or other mark
to show that VARIABLE VARIABLE
the value of RES|STOR CARAC|TOR the
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Chapter 6: Reading a Schematic 139
For a refresher course on the concepts of capacitance and inductance, see Chapters 4 and 5.
Photo-Sensitive Components Help You See the Light
Special light-dependent versions of resistors, diodes, and transistors react to changes in illumination. The value of the component varies depending on the amount of light that strikes it. Most schematics show the light-sensitive nature of the component by using one or two arrows pointing into the body of the component.