Radio Shack SSM-1750 User Manual download pdf (Page 49)

Q2 = A Q2

is on

when A is

not high but low.

LED4

02

1 =

A OR

C Q1

is on when either A or C

is high.

V;

LED3,

5

03

=

B OR C Q3

is

on

when

B or C

is high.

LEDI,

7

Q4 = A AND

B Q4 is on

when A and B are high.

V+

R4

LED2,

6

Fig. 3.

The logic expressions

and

electronic

equivalents

for each driver

transistor.

from either input.

The AND

expres-

sion

follows

from the fact that Q4

will

be turned on

by R4 only

if A

AND C are

high

simultaneously.

As mentioned above, U3 must be

reset after the sixth pulse. Otherwise,

it would

count

right

on up to

16

and

cause problems

with

counts coming

out of turn.

The

sixth

pulse causes

the B and

C outputs

to both go high.

This

condition

is

detected and

used

to

reset

the counter.

If

either

the B or

the C output

is low, the reset input is

also low. However, when both

B

and

C are high, the counter automatical-

Iy resets to zero.

Construction

There is nothing particularly critical

with

regard to circuit layout.

For

Fig. 4. Actual -size

etching

-and

-drilling

guides for both sides of printed -

circuit board.

convenience, though, printed- circuit

board construction

is recommended.

You

can

fabricate this double -sided

pc board, using the actual

-size

etch-

ing- and -drilling guides

in Fig. 4. If

you

do, make

sure

that both guides

are properly

registered on the pc

blank during exposure.

When in-

stalling components on a

home-

made double -sided pc board, solder

all leads that contact the top and

bot-

tom traces on both

sides of the

board. Additionally,

it

is

necessary

to solder short

lengths

of solid

hook-

up

wire

or component

lealds in the

holes

that

bridge conductors on both

sides of the board.

If you prefer

not to etch and drill

your

own board, you can

purchase

a

kit of parts that contains a

ready

-to-

wire

pc board

from

the source given

in

the

Note

at

the

end

of the Parts

List. The board in this kit has plated -

through holes that eliminate the need

for having to solder on both sides.

Referring

to

Fig. 5, begin

wiring

Say

You Saw It In Modern

Electronics

September

1986 /

MODERN ELECTRONICS / 55

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