Just For Starters
SW10
N.O.
SW4
N.C.
SW3
DPDT
SW1
Figure 6. Switches and Push Buttons.
General
Diode
D10
1N914
Zener
Diode
D5
1N5333B
3.3V
LED
D1
Green
NPN
Q1
2N3904
PNP
Q2
2N3906
Figure 7. Diodes and Transistors.
symbols for a small-signal diode, an
LED (light emitting diode), a Zener
diode, and NPN and PNP transistors.
As with other components, electrical
ratings (e.g., power, voltage, current)
may be included in the diagram.
Diodes and transistors use “Dx”
and “Qx” reference designators,
Integrated Circuits
Figure 8. LM555 Symbol.
8
4
respectively. Symbols often include
detailed part information — such as the
component part number — because
there are wide ranging characteristics
between different diodes and transistors. Understanding the function of a
diode, transistor, or semiconductor in
general goes beyond the basic skill of
reading a schematic diagram. You
need familiarity with the component
and how it behaves in various
configurations.
3
5
VCC
RESET
DISCHARGE
LM555
U1
OUT THRESHOLD
2
CONTROL TRIGGER
GND
7
Figure 9. 74LS00 Break Out.
6
1
VCC 14
1
2
74LS00
1/4 U2
3
10
9 74LS00
3/4 U2
8
GND 7
4
5 74LS00
2/4 U2
6
13
12 74LS00
4/4 U2
11
Schematic representation styles
for ICs are quite numerous due to the
great diversity among ICs. General
ICs are drawn as a rectangle with
multiple pins on one or more sides.
Each pin has both a signal name and
a pin number. Figure 8 shows a symbol for an LM555 timer IC. Simple
logic ICs may break out individual
gates for clarity, as shown in Figure 9.
Note that the 74LS00’s power pins
(VCC and GND) are drawn on the
first of four NAND gates that comprise the IC. Like other components,
each IC in a schematic diagram has
an associated reference designator.
IC reference designators are
generally in the format “Ux.”
It’s Just
Documentation
F
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c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
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g
Keep in mind that schematics are
just another form of documentation
and, therefore, styles vary according
to the context and intent of the
designer. While certain institutions
may have standards, there are no
universal rules for how to draw a
schematic diagram. The main goal to
keep in mind when drawing a
schematic diagram is clarity.
Feel free to insert descriptive
text or sketch a picture to convey
accurate information to your intended
audience.
There is a vast collection of
electrical components in the world
and each item’s symbol may look a
little different from others. Therefore,
you learn to read schematic diagrams
by diving into a particular circuit and
trying to figure it out. You’ll gain
“schematic literacy” after doing this a
few times. NV
26
Circle #37 on the Reader Service Card.
MAY 2004
