Radio Shack SSM-1750 User Manual download pdf (Page 52)

llIiI/

ELECTRONICS

NOTEBOOK 11111111111111

Forrest M. Mims III

Piezoelectric

sound sources

have

been

available

for more

than 20 years.

In the

mid-

1960's, Mallory

introduced

the Son

alert', a piezoelectric

sound source

in-

stalled

in a plastic

housing

with

a built

-in

driver circuit.

Today

numerous

kinds of

self -contained

piezoelectric

buzzers,

they are

often called,

are

readily avail-

able

from

many sources.

Radio Shack's

catalog,

for example,

describes

piezo-

electric

buzzers

that produce

continuous,

pulsing,

dual, and

chime sounds.

Consumer

and

industrial applications

for piezoelectric

buzzers abound.

They

are used

wrist

and

pendant

watches,

al-

arm

clocks,

calculators,

microwave

ovens,

telephones,

smoke

alarms, geiger

counters,

automobiles,

and

in many

other consumer

appliances

and

industrial

equipment.

Because piezoelectric

buz-

zers and elements

are

inexpensive,

easy

use,

and

highly reliable,

they are particu-

larly popular

with experimenters.

Piezoelectric

Buzzer

Design

Piezoelectricity

literally

means

pressure

electricity.

It is the phenomenon

wherein

voltage

produced

when pressure

applied

to certain

crystals.

Conversely,

piezoelectric

crystals

change their

shape

response to

an applied

voltage.

Crystals

of quartz,

tourmaline,

and

Rochelle salt exhibit

piezoelectric

pro-

perties.

Though

its

piezoelectric

effect

weak, quartz

is the most

important

these

crystals.

It is used

make highly

reliable and

reproducible

mechanical

os-

cillators

whose

frequency

varies

only

slightly

with

changes

in temperature.

Such

crystals provide

frequency

stand-

ards

and

highly stable

oscillators

for digi-

tal clocks

and

watches,

television

receiv-

ers,

radio transmitters,

and so

forth.

Rochelle

salt

crystals

have long

been

used to

make

high- impedance

crystal

ear-

phones and

microphones.

Development

high-

intensity piezoelectric

buzzers

and

devices awaited

the

develop-

ment of

family of

manmade

ferroelec-

tric

ceramics

having excellent

piezoelec-

tric

properties.

Among the

best

deve-

loped

of these

zirconate titanate.

Piezoelectric

Buzzers

METAL

DISC

ELECTILIC

CERAMIC

CompvcTIVE FILM

COLOR

CODE

1 -

BLACK

2.- RED

3 -

BLuE

sTaIJDARD

W iTN

FEEDBACK

TAß

Fig.

1. Details

typical piezoelectric

buzzer elements.

Figure

shows

two typical

piezoelec-

tric sound

generator

elements.

Most such

elements

have a

diameter

ranging

from

25 to

40 mm and a preferred

or resonant

frequency

from 2

kHz to

7 kHz. The

elements

are

formed by bonding

a disc

piezoelectric

ceramic

to a

larger disc of

flexible

metal. Electrical

contact

to the

disc is provided

through

the

metal disc

and

a conductive

film applied

to the

op-

posite

side of the

ceramic

disc.

When a

voltage

applied

across

the two

electri-

cal

contacts,

the diameter

the ceramic

disc expands.

This

motion

is transformed

into a

movement

perpendicular

to the

plane of the

disc by the

spring -like

action

the

metal disc.

The direction

mo-

tion

of the

disc is dependent

the polar-

ity of

the applied

signal.

Note that

one of the

discs

in Fig.

1 is

equipped

with a third

terminal

called the

feedback tab.

This tab

is a separated

por-

tion

of the

metalized coating

the sur-

face of the ceramic

material.

When an

os-

cillating

signal

is applied

across

the other

two

terminals,

voltage appears

this

tab

that can

be used

to stabilize

the

fre-

quency

of the oscillator

circuit.

about this

later.

circular

disc

will tend

vibrate at a

preferred

frequency

called

the

mechani-

cally-

resonant

frequency.

A dramatic

il-

lustration

of the

resonance

a piezoel-

ectric buzzer

element

can be arranged

connecting

the element

variable

fre-

quency

signal

generator,

either

a corn-

Fig.

2. Nodal

pattern

of an oscillating

piezoelectric

buzzer

element.

merciai unit or

one of the circuits

that

follows.

The

element

then

placed

with

its metal side up on

a soft surface (a

few

layers of tissue

paper

works well), and

some light, dry

powder such as sugar

artificial sweetener

is then poured

the center

of the metal

disc. A signal

then applied, and

its frequency

is swept

across the audio

spectrum.

The powder may

vibrate slightly

when

the signal

is applied.

At a particular

fre-

quency,

the powder

will

become

consid-

erably

agitated.

Much of the pow-

der

will vibrate

off the surface

of the disc,

leaving behind

a nearly circular

pattern

outlining

the

disc's nodal

region, the po-

sition of

minimum

vibration.

Figure

shows a

typical powder

pattern

that re-

sults

when a resonant

signal

applied

a real

disc.

60 /

MODERN ELECTRONICS

/ September

1986

Say

You Saw

It In Modern

Electronics

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