Elenco Electronics MO-1251 Instrukcja Użytkownika Pobierz pdf (Strona 56)

100

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FIG.

4 -THE AUTHOR'S PROTOTYPE WAS ASSEMBLED

ON A SOLDERLESS

BREAD-

BOARD. Use

insulated hookup wire to minimize the

possibility

of shorts.

the

BCD

count from IC9 and ICIO.

Any further

increase in transducer

voltage will

tracked, displayed,

and maintained.

Of course, the transducer is not yet

calibrated. Calibration implies com-

parison with

known standard.

Get-

ting into the mathematics of

footcandles

can be pretty compli-

cated,

so we'll

develop our

own stan-

dard-

albeit crude for our

light

sensor. As shown in Fig. I, simply

punch holes of increasing

diameter

into a cardboard

disk

to allow

extra

light to pass

through each

successive

hole;

then

chart

the light intensity fall-

ing on the LDR, for

each

punched

holè, by measuring

the

voltage

across

and the display's reading. The

more light that falls on the LDR, the

lower its resistance

and, con-

sequently, the greater the

voltage

drop

across

R1. Although

that

method is far

from scientific, it should give you a

rough scale with which

to compare

lilght levels. The

same

technique can

be used

with

other transducers, such

as temperature- dependent resistor's.

Using a thermometer

for

comparison

can

give

a more meaningful calibra-

tion curve.

Figure 3 is the complete

schematic

of the digital -readout peak- detector.

As shown in Fig. 4, the author used a

proto -board for assembly, but you

may just as easily use a prototype

PC-

board and wire-wrap all connections.

Testing

If your

circuit

fails

to respond after

construction, use the following trou-

bleshooting procedures

to pinpoint

the problem (refer

to Fig. 3):

Switch SI to the test position, and

adjust the

potentiometer R41 for an

output

of 2.5

volts.

Check

TP3 for

a 30 -Hz clock sig-

nal. No clock on TP4 means

there's a

problem

with

either

IC2

or IC3. The

clock signal at TP5

should have a

frequency

of about l Hz. Test Point

TP8 sould output less

than

one

clock

cycle every 16 seconds.

3. If all checks out so far, press

and

release the reset button (S2)

and see if

TP6 goes from

low

to a high, and

then back to low. If

there's a problem,

check Q2 and associated

circuitry.

4. Check the output of the R2R ladder

network at TP7.

To do that, press and

release

S2 to reset the counters, and

watch TP7

(using an oscilloscope or

meter) for

a slow rising DC

voltage

( +

5 -volts maximum). Any

deviation

from

the normal ascension indicates a

problem in the R2R ladder network

binary

counters

IC4

and IC5. Note:

The resistors in the R2R ladder, R10-

R19, should be within 1%

tolerance.

The

final test is to simultaneously

press and release

SI and S2, then

wait

for the LED's

to stop counting.

When

they stop, the display will register a

number equivalent to the 2.5

volts

R4I potentiometer. (Don't worry

about the actual displayed

number.)

Repeat that procedure a few times.

The same number should re- appear

on each test. Next, turn the test poten-

tiometer R4l up to three

volts.

The

LED's should start counting up to

some number

and then stop.

R -E

1 2 ... 51 52 53 54 55 56 57 58 59 60 61 ... 99 100

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