chatter(the sequential opening and closing of contacts before final closure). Under lowlevel
loads, mercury-wetted reed contacts provide consistent and predictable resistance
over wide ranges of temperature and contact load current. But mercury slows contact closing
to about 2 ms. These relays are position sensitive and must be mounted in an upright
position to keep the reed contact surfaces immersed in the mercury.
Electromechanical Switches
Electromechanical switches for electronics are small mechanisms with electrical contacts
that permit manual opening or closing of circuits. The simplest and most basic of control
devices, they are made in many forms and styles. They are packaged for panel or circuitboard
mounting. A typical switch has movable metal contacts controlled by a lever-actuated
spring mechanism. The contacts are insulated from the actuator by an insulating case.
Spring action accelerates shutoff to minimize arcing.
Figure 3-3 shows a cutaway view of a typical miniature switch. Many of these miniature
switches are scaled-down versions of electrical power switches. The most popular miniature
and subminiature switches for electronics are: (1) pushbutton, (2) toggle, (3) rocker,
(4) slide, (5) rotary, and (6) thumbwheel.
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PUSHBUTTON, TOGGLE, AND ROCKER SWITCHES
Pushbutton, toggle, and rocker switches are generally made for both panel and circuitboard
mounting. Some have threaded bushings for mounting through a hole in a metal or
plastic panel with a lockwasher and ring nut. Others are mounted with snap-in bezels. The
switch is inserted through a cutout and anchored with a snap-in clip. Many of these
switches have been adapted for circuit-board mounting so that the actuator (pushbutton,
handle, or rocker) projects through a hole in the panel or cover. These switches are mounted
on the circuit board or card, which provides mechanical support. Pushbuttons, levers, and
rockers are made in an almost infinite variety of colors, shapes, and sizes to meet specific
customer requirements.
Pushbutton switches are combined with volume controls on dual-purpose switches.
Others function as momentary key switches. Some circuit-board switches for “set-andforget”
encoding functions are mounted within the host product and are inaccessible to the
user.
Switch ratings depend on their size, their contacts, and the dielectric properties of their
cases. In addition, the spacing between conductors and contacts is considered. International
safety standards apply to some switches rated for 50 VDC or more, but they are usually
required for those rated 120 VAC or more. Panel switches for electronics are commonly
rated for 5 A at 125 VAC or 3 A at 220 VAC.
Some pushbutton and rocker switches can be illuminated with incandescent lamps or
light-emitting diodes (LEDs) but the term illuminated pushbutton (IPB) switch refers to
families of industrial-grade switches with square or rectangular end caps that function as
indicators or annunciators.
SLIDE SWITCHES
A slide switch is a switch assembly that includes a sliding element which can make or break
internal contacts. Lateral movement of the slider can turn an external circuit on or off.
These switches are made in many different sizes, ratings, and styles. They are used in
radios, electronic test instruments, and home appliances.
ROTARY SWITCHES
A rotary switch is a switch assembly that includes an axial shaft on which a movable contact
is mounted. Step rotation of the shaft causes the movable contact to make or break with
internal fixed contacts, turning an external circuit on or off. Multideck versions contain
multiple stacked wafers or decks, each with fixed contacts, and an equal number of shaftmounted
movable contacts. They permit the simultaneous switching of multiple electrically
isolated circuits or functions. Typically panel mounted, rotary switches are used in radios,
electronic test instruments, home appliances, and automobiles.
THUMBWHEEL SWITCHES
Thumbwheel switches are encoders that convert numerical settings into binary digital code.
They are typically made as stacked assemblies of two or more modules for panel mounting.
ELECTROMECHANICAL SWITCHES 57
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Each module contains a thumb-actuated wheel marked with the numbers 0 to 9 and a codeconversion
circuit card. When powered and set to a desired number, the binary equivalent
signal appears on its output pins. Thumbwheel and similar lever switches are used to enter
commands into digital circuits for instrument, machine, and process control.
SWITCH CONTACT ARRANGEMENTS
Commonly used switch contact arrangements are illustrated in Table 3-1. The choice of
switch contact material depends on electrical rating and anticipated applications. Silver
alloy is used for general-purpose applications and gold-flashed silver alloy is used for low
voltage. The contacts in many switches are spring loaded to accelerate the switching action.
(Slow switch closure causes contact burning, sticking, and arcing.) The spring also provides
a tactile and sometimes an audible response to indicate that the contacts have closed.
Very low voltage circuits are called dry circuits. Microampere currents are blocked by
resistive oxidation or contamination films on the contacts, so they are lightly gold plated or
flashed to prevent this contamination buildup. It is particularly useful if the switch is inactive
or in storage for long periods.
All switches are rated for contact resistance, working current and voltage, dielectric
strength, insulation resistance, lifetime, and ambient temperature range. Switch contacts
arederated for high ambient temperatures regardless of their electrical ratings. Contacts
can be inadvertently welded together in DC circuits, but AC reversals minimize this effect.
As a result, DC switch ratings are lower than AC switch ratings.
Switch cases are molded from various thermoplastic or thermosetting resins or ceramics.
Flame-retardant thermoplastics cost less than the thermosetting plastics such as diallyl
phthalate or phenolic. The metal parts of panel switches typically are formed or stamped
from sheet steel, stainless steel, or copper alloys such as phosphor-bronze or berylliumcopper.
Special Switches
REED SWITCHES
A reed switch is an assembly of a reed-switch capsule and a permanent magnet, as shown
in Fig. 3-4. The magnet is mounted so that it moves with respect to the reed-switch capsule.
When close to the reeds, the magnetic field penetrates the capsule, causing the reeds to
open or close, thus switching the load on or off. The response depends on the contact
arrangement of the reeds. Unless otherwise stated, the switch capsule is assumed to be a
dry reed switch.
MERCURY-WETTED REED SWITCHES
A mercury-wetted reed switch is a switch that includes a mercury-wetted reed-switch capsule
containing a small amount of mercury to improve performance, as in mercury-wetted
reed relays. For further information on the mercury-wetted switch capsule, see “Mercury-
Wetted Reed Relays” in this section. These switches, like the relays, are position sensitive
and must be mounted in an upright position to keep the reed contact surfaces immersed in
the mercury.
HALL-EFFECT SWITCHES
A Hall-effect switch is a magnetically actuated momentary keyswitch that contains a Halleffect
transducer(HET) and transistor amplifier with trigger circuits integrated on a silicon
chip and a small permanent magnet. When the IC is powered by a DC current and the spring-
SPECIAL SWITCHES 59
Figure
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stores, travel agencies, financial institu tions,
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and airline terminals, and even in the passenger sections of airliners. Some optoelectronic
displays have reached monumental billboard size in cities, where they provide news
bulletins as colorful, moving “zippers.” The latest automobiles now include three or four
different kinds of indicators and displays, and more are in the offing.
Of all the display technologies, liquid-crystal displays have shown the most conspicuous
improvement within the past 10 years. They first appeared as black segmented digits on
white backgrounds in watches and calculators, but they have now morphed into multicolor
flat panels. With increasing resolution and wider color palettes, LCDs are now challenging
CRTs as the leading computer monitor displays and they are now in TV receivers, camcorders
and digital cameras.
The charge-coupled device (CCD) camera is also replacing traditional video camera tubes
inside and outside of TV studios and in industrial and consumer still and video cameras.
The standard observer curve, shown in Fig. 13-1, is helpful for understanding the relations
CONTENTS AT A GLANCE
Overview
Liquid-Crystal Color Video Displays
Cathode-Ray Tubes (CRTs)
Electroluminescent (EL) Displays
Light-Emitting Diodes (LEDs)
Electrophoritic Displays
LED Lamps
Plasma Display Panels (PDPs)
LED Alphanumeric Displays
Vacuum-Fluorescent Displays (VFDs)
Liquid-Crystal Displays (LCDs)
Holography
between wavelength and color distribution in the visible-light band and its effect on
human vision. The curve shows that the human eye is most sensitive to yellow-green at 555
nm but acuity trails off in the violet and deep red regions. This curve indicates that dim
sources of yellow and green light can be more eye-catching than bright sources of red or
violet light.
This section focuses on devices, circuits, and systems that function in the visible-light
realm while Sec. 12, “Optoelectronic Components and Communication,” concentrates on
nonvisible optoelectronics.
Cathode-Ray Tubes (CRTs)
The cathode-ray tube (CRT) is a glass vacuum tube containing one or more electron guns,
a phosphor screen to convert the energy of the electron beam into visible light, and provision
for the electrostatic or electrodynamic control of one or more electron beams. It
remains the most popular and cost-effective display today for desktop computers, TV
receivers, and electronic test instruments despite the fact that it was invented well over a
century ago.
There are two basic kinds of CRT: electromagnetic and electrostatic. These tubes are further
classified by color as black and white or monochrome and multicolor, and by the number
of electron guns they contain. Computer monitors, TV receivers, and many kinds of
scientific and medical apparatus include multicolor electromagnetic CRTs, while monocolor
electromagnetic CRTs are still prevalent in radar and sonar systems. However, such
electronic test equipment as oscilloscopes and spectrum analyzers contain monocolor electrostatic
CRTs because of their higher resolution and suitability for presenting rapidly
changing waveforms.
ELECTROMAGNETIC-DEFLECTION CRTs
The electromagnetic- (EM-) deflection CRT, as shown in the diagram of a monocolor version
Fig. 13-2, is designed for the installation of external electromagnetic coils around its
neck for the control of the electron beam. Electrons are driven from the cathode, which has
been heated by a filament, and they are formed into a beam by the control and screen grids
of the electron gun. The beam is focused at the back of the phosphor-coated faceplate and
is drawn to it by a potential as high as 2500 V on the anode. While transiting the length of
the envelope, the beam passes through the magnetic fields of the focus and deflection coils
that together form a magnetic lens capable of deflecting the beam in the x and y directions.
The beam can be scanned in a raster sweep format for receiving TV images or it can be
formed into a radial scan for a radar plan-position indicator (PPI) display. The phosphors
on the faceplate cause the beam trace to persist long enough to form a visual image.
Color CRTs, operating on the same principles, typically contain three electron guns as
shown in Fig. 13-3a. One is assigned to each of the colors (red, green, and blue). In a multicolor
tube, the phosphors are grouped as dots or bars arranged in patterns to form pixels.
A single beam impacting on a single phosphor dot, for example, creates a single color spot.
Two dots activated simultaneously result in a mixed color, and the simultaneous illumination
of all three dots produces white. A black spot appears if none of the dots in the triad is
illuminated.
The three electron beams are coordinated to move synchronously and, because of their
spatial separation, the beams pass through a shadow mask at slightly different angles to
CATHODE-RAY TUBES (CRTs) 249
illuminate their assigned phosphor dots. The mask is a thin sheet of metal mounted just
behind the phosphor-coated back of the faceplate. Holes in the metal sheet correspond to
each phosphor dot on the faceplate, allowing the assigned electron beam (red, green, or
blue) to strike the correct color phosphor. Figure 13-3b shows the phosphors deposited on
the faceplate in triangular groups of three dots, or triads, but other CRTs have the phosphors
arranged in vertical stripes.
For further information on how the multicolor electrodynamic CRT works in a television
receiver, see Sec. 19, “Television Broadcasting and Receiving Technology.” The main difference
between color CRTs for computer monitors and those for TV receivers is the RGB
phosphor pitch. At present the 0.28-mm pitch is becoming the standard for personal computer
monitors, but computer workstation monitors feature 0.20- to 0.26-mm dot pitch. For
further information on computer monitors see Sec. 16, “Computer Peripheral Devices and
Equipment.”
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