8/15/2019 Frequêncimetro Dgital (7226B)

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with good accuracy at low

frequencies

Intersil

IC

type 72268 is just the right

counter for a simple but reliable fre-

quency meter which covers a range

of 9 MHz. The circuit of the meter

divides into four functional sections:

. input stage, Tl, T2, N2, N3;

. multiplier, FF1,FF2, IC3, IC4;

. counter, IC5;

. display,Ldl . . . 6

In general, the circuit is a standard

design, much of which has been

described in earlier issuesof Elektàr.

The primary function of the input

Q

N1 . . .N4 =

2/3

IC1 = 74LS04

FFl.FF2 ~ IC2. 4013

 0

7 44

stage is converting the input signal

into rectangular pulses that are fed

to the counter either direct ar via the

multiplier. The stage can handle in-

put voltages of up to 50 V r.m.s.

which is sufficient for most

measurements. Diodes Dl and D2

conduct when the input voltage is

above about 600 mV so that the in-

put impedance is determined primar-

ily by the value of R2, that is, around

1 M.

The multiplier (x 100) is particularly

important for the measurement of

frequencies between 5 Hz and

1 . . . 2 kHz.

The counter, the Intersil 72268, con-

~

july/august 1984

tains a crystal

oscillator, a time base,

a counter, a seven-segment decoder,

a multiplexer, and a number of

drivers for the direct contrai of the

LED display.

In our prototype a 1 MHz c rysta I was

used for driving the on-chip oscil-

lator, but if D5 is omitted a (cheaper)

10 MHz crystal may be used.

The LED display is the popular type

MAN 4640A.

The function of the switches is:

. Sla connectsthe inputstage to

the counter either direct or via the

multiplier (as shown);

. Slb ensures the correct position

of the decimal point when the

multiplier is in circuit;

. S2 normally determines the pos-

ition of the decimal point, that is,

whether the display reads kHz or

MHz;

. S3 is the mains on/off switch;

.

S4 is the reset switch;

. S5 serves to test the display:

when it is pressed, ali segments

should light.

Finally, note that printed circuit

84462 for the meter has no provision

for the display; this may be fitted on

board 80089-2 originally designed for

the Junior Computer. til

r ~

 

. LD 1

...

LD6 - MAN 4640A .

a 4 LD1 LD2 LD3 LD4 LD5 LD6 :

b13 a a I

c .

 

f ; b

 

; b I

.6- - - - -

I

~ ~  _ oiI /~

 _ _ oiI /~

1

p 9 dp

Q

0

IC5

72268

(lntersil)

dP,30

* -....

 ao.

84462

8/15/2019 Frequêncimetro Dgital (7226B)

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5emíconductors:

D1,D2,D4. . . D6 =

1N4148 for D5 see text

D3 = LED red)

D7

...

D10 = 1N4001

T1 = BF 256A

T2 = BF 494

T3 = BC 547

IC1 = 74L504

IC2 = 4013

IC3 = 4046

IC4 = 4518

IC5 = 7226B lntersil)

IC6 = 7805

Ld1

. . .

Ld6 = MAN 4640A

 common cathode

july/august 1984

  rts list

Resistors:

R1 = 1 k

R2 = 1 M

R3,R6 = 470 Q

R4 = 220 Q

R5 = 2k2

R7,R12 = 4k7

R8 = 18 k

R9 = 330 Q

R10,R24 = 100 k

R11,R21

. . .

R23 = 10 k

R13

. . . R20= 10Q

R25 = 4M7

Capacitors:

C1,C8,C11 . . . C13 =

100 n

C2 = 100 ,/16 V

C3,C6 = 10 ,/16 V

C4 = 22 p

C5 = 1000 ,/16 V

C7,C9 = 39 p

C10 = 40 p trímmer

Míscellaneous:

51 = double-pole change-

over switch

52 = single-pole change-

over switch

53 = DP5T switch

54,55 = spring-Ioaded

push-button press-to-make

switch

X1

= 1 MHzor 10MHz

crystal  HC18 or HC25

holder) see

text

Tr1

=

mains transformer,

secondary 9 V /500 mA

F1 = fuse, 100 mA, delayed

action

prínted circuit

84462 fre-

quency meter less display)

printed circuit 80089-2 for

the dísplay)

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