Elenco Electronics MO-1251 Manual do Utilizador descarregar pdf (Página 53)

L J

S -J

DIGITAL

ROGER D.

SECURA

DETECTOR

When you need to know the hottest, the fastest, the

highest,

or

the

absolute

most,

you

need to build

our

peak

detector.

WHAT'S THE TOP WIND SPEED DURING A

hurricane? What about

that jumbo

jet

on final

approach, flying 1000

-feet

over your home: Is the noise

pollution

higher than that allowed by law? How

hot

does beach -sand get under a

blaz-

ing

summer

sun?

To

answer those questions,

you

have

to measure the relevant physical

parameter, store the maximum

event,

and then display the result. To sense

the relevant parameter, you need a

transducer. To track and hold the

maximum

event, you

need

a peak de-

tector. And to record the result, you

need a digital

display.

Such a peak -

detecting

device

should

continuously

track, hold, and display the maximum

level of any physical parameter; for

example, speed, loudness,

tem-

perature, pressure, position, flow

rate, force, light intensity,

and

so

on -you name it.

Transducers

Getting the

world

of electronics to

communicate with the physical

world

is like trying

to

mix oil with water -

an almost impossible task, unless you

have the right

emulsifier.

We know

that

emulsifiers

work with

oil and

water,

but what works with physical

quantities

and electronics? You can't

shake

them up in a bottle. To get them

to mix you need a transducer.

And

their are literally

hundreds of different

types

of transducers; each type mix-

ing a

specific

physical

parameter

with

electricity.

A transducer outputs an electrical

signal that is proportional to the

mag-

nitude of the physical event it's detect-

ing; an output can be a series of digital

pulses, an analog

voltage,

a

varying

frequency, or a change

in current or

resistance.

An

example

of a practical trans-

ducer is a Light Dependent Resistor or

LDR, which is a resistor

whose

resis-

tance changes

in proportion to the

amount 01 light

striking its surface.

(Cadmium

-sulfide photocells are

the

most

common

LDR's.) But our

peak

detector can

sense only voltage

within

the 0 to 5 -volt

range; it can't sense

resistance

at all!

What's

needed is a

method to convert the LDR's resis-

tance to an equivalent

voltage. A

typ-

ical

LDR

might have a light -to -dark

resistance range of 100 ohms to

500,000 ohms. A circuit

must

be de-

signed that transforms that

resistance

range into a

voltage range between 0

to 5 volts. That conversion process is

called signal conditioning.

As shown in Fig. I, to condition

the

LDtt1

TO IC1

PIN 12

+5v

CARDBOARD

DISK

(DIAMETER

OF HOLES VARY

FROM PIN HOLE TO 11

DISPERSED LIGHT

R1

10K

r

FIG.

1 -THE

LIGHT DEPENDENT RESISTOR (LDR1) IS A TRANSDUCER

whose

resistance

changes in proportion

to amount

of

light falling

on

its

surface.

59

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