Elenco Electronics MO-1251 Manual do Utilizador descarregar pdf (Página 54)

60

PARTS

LIST

All resistors are 1/4-watt, 5% unless

otherwise

noted)

R1, R7 -1000

ohms

R2 -100

ohms

R8, R9, R42- 10,000

ohms

R3-R6, R10- R19-

10,000

ohms,

1%

R20

-R40 -330 ohms

R41- 250,000 -ohm

potentiometer

Capacitors

C1

-2211F, 16 volts, electrolytic

C2

-10µF, 16 volts, electrolytic

C3- .01µF,

50 volts, ceramic disc

Semiconductors

D1, 02 -lN914 Diode

DISP1- DISP3 -7 Segment

LED Dis-

play (common anode)

Q1-

2N3906,

PNP

Transistor

LDR, a

voltage

divider

is formed by

connecting the LDR

in series

with

resistor

R1

and a 5 -volt

source.

When

the

light source is maximum, the

LDR appears as a

low

resistance,

al-

lowing almost the entire 5

volts

to be

developed across

R1.

When

the light

source is minimum (dark), the

LDR

has its highest resistance, so almost

TEMPERATURE

CPRESSURE

VELOCITY

J

LIGHT

INTENSITY

TRANSDUCERS

Q2-

2N3904, NPN

Transistor

IC1-

LM324,

Quad

Op -amp

IC2

-555,

Timer

IC3

-4066, Quad

Bilateral Switch

IC4,

IC5-

74193, 4 -bit up -down

counter

IC6

-IC8-

74190, 4 -bit up -down

counter

IC9 -IC11

-7446,

BCD-to- Seven-

Segment

Decoder /Driver

Other components

S1, S2-

Normally -open momentary-

on

push

button

S3-6 position

rotary

switch

LDR1 -cadmium- sulfide (CdS) pho-

tocell

all

the

voltage

is developed across it,

and practically no

voltage

is de-

veloped

across Rl. So far,

we've

taken a

light

-intensity transducer,

the

LDR,

and conditioned

its changing

resistance to be compatible

with

the

our peak

detector's

input requirement

of

0 to 5-

volts.

Shortly,

we'll see how

to calibrate our transducer.

PEAK

VOLTAGE

IOC)

FROM

TRANSDUCER

STORED IN

CAPACITOR

REFERENCE VOLTAGE

FROM PEAK DETECTOR

I

PEAK

DETECTOR

=

OUTPUT FROM

TRANSDUCER

IS

A FLUCTUATING

DC

VOLTAGE

1 +5V

MAXIMUM)

BUFFER

V

N

COMPARATOR

BCD

COUNTER

BCD

DECODER

1 1

1I

CLK

How

it works

Figure 2 shows

a

block diagram of

our digital- readout peak- detector.

The

purpose

of the

peak detector

is

to

track

and hold

(using the charge -stor-

ing ability of a capacitor) the highest

output

voltage

from a transducer. An

op -amp

with

a high input -impedance

is used as a buffer to

ensure

that the

stored

charge

on

the capacitor doesn't

leak off. Another op -amp is used as a

comparator that has the task of en-

abling

/disabling

the Binary Coded

Decimal (BCD)

and binary counters.

Initially, the

voltage

on the invert-

ing input of

the comparator

is

at

ground level. As a small

voltage

(0-5

volts)

is captured by the peak detector

and presented to the comparator's

non -inverting input,

the output

will

swing high, which asserts

the bilat-

eral switch; clock pulses now

pass

through the

switch

to clock both the

BCD and binary counters. The out-

puts of the binary counters are con-

nected to

a

R2R ladder

network,

which

functions as a digital -to-

analog

converter. As the binary count in-

creases, the

R2R ladder voltage

also

CLK

BILATERAL

SWITCH

BCD

COUNTER

BCD

DECODER

LED

DISPLAY

1

1A^ AAA,

CLK

BINARY

COUNTER

Off

BINARY

COUNTER

*f#

- *

R2R LADDER

ID

A)

FIG. 2 -THE TRANSDUCER, PEAK DETECTOR,

AND DISPLAY are

the main components

of a digital

-readout peak-

detector.

1 2 ... 49 50 51 52 53 54 55 56 57 58 59 ... 99 100

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