318

Sinking characteristie, Observation

D |/4 74C00
0- 50 mA

((-5
V;

5V
5V 750 Q

Input characteristics Observation

V;
|/4 74CO0

SV
750 s2

4.33 SOLVED EXAMINATION QUESTIONS

1. Set up adigital circuit to light four LEDS ina row. The lighting pattern is first and second,
second and third. third and fourth, fourh and first with an interval of one second. This must
repeat continuousiy.
This can be achieved using a ring counter with 11 pattern rotating. For that, set up a ring
counter and preset any two neighbour flip flops by connecting PRESET terminal to logic 0.
After its usage keep PRESET pins open. Apply clock pulses.
2. Experinenially find out thc propagut1on delay, sourcing and sinking characteristics of a
CMOS NAND gate. Draw the transfer characteristics.
3. Design a combinational circuit whose input is a 3-bit number and output is its 2's comple
ment.
Write the numbers and corresponding 2's complement in the truth table and simplify
using Karnaugh map. Realize the circuit using NAND gates.
4. Obtain IkHz square wave from a 4 kHz square wave.
Set up a divide by 4 counter using two JK flip flops in cascade as in the ripple counter.
 set up adigitalcircuit to obtain the
following waveform from a grven ciock puske irain

Output

This can be obtained by a Johnson counter using three f1p floos hec atise the
requre
duty cyclc is 172. Same output can be ohtained using a rne counter with ill otafing. But it
necqs 6flip flops.
Sct ip a BCD Counter to count up to 99 using suitable Cs
Usc two decade counters using two 7490 in
cascade
Design ang set up amod-6 counter using JKFF so that if any unw inted state occurs the nex
clock pulse will reset the counter.
Set up an asynchronous counter using three flip flops in the state diagram next states
of the counts 5,6 and 7 should be ).
A
four variable Boolean function is given as f( A. BC D) =El 2.4 5.9 10. 1. i5 D
10, 11). Rcalize a circuit to obtain the function
Zimplies that the numbers are minterms. In other words it is d sum of prOduct expresso
D stands for don't cares. Make a Karnaugh map and enter Is and Xs respectively. ven
after 2and D in the columns corresponding to the numbers Mimm1ze the funciion sing k
map and implement it using logic gates.
=I0. i 4. 5.9 0.15 D.11
Afour variable Boolean function is given as f(A. B. (CD)
Realize a circuit to obtain the function.
rdu! o sum epeson.
Ilimplies that the numbers are maxtems. In otter wOrds, it is
a Karnaugh map and erter s and respevuvety. gven
D stands for don't cares. Make
and D in the columns corresponding to the nnhers Minammze the tuntx sing k
after II
gates.
map and implement it using logicgenerator
set up a 4-bitparity using oni \ IND at
I0.Design and &maps.
cither odd or even parity generator with the hel ot uth tabl ai
Sct up s (RO.
register and observe the waveloMs ai \ars
II.Set upa four bit shift
clxk
liecd high frequency signal as
an LED when a key is prevwdeN nes
2.Set up a circuit to light
ooopulse utley th ln ner iN e S
The LED should glowwhen the eniniY aivINhrvus If ts
mod-5 ipple counter
Thiscan be achieved by a Ust be desNNtocear
combinational circuit
asynchronoUs, a !ID
Q, Q, should he ANDed und cometdiann'NWth thptles
steps to l01. achieved by a shitt regst
This can also be
N i pul al
Genele a HZ pulse signal and covet it into
13 ibeMleng gate 15 thmer)
Geue. 2MZ square wave usug :aabielt g counter
divvide by circuit atiehehiCt h tps
p
itloa (usmg
andfeed llpsor,Johnson t pt h e t e t Is notpterrel because
using4 lp Coter uN
320

we need four flip flops.

14, Dèsignamod-5 conter using 7490 and display its output using 7-segment display.
Use 7490, 7447 and FND 507.
15. Set up acircuit to divide a square wave by 8.
U'se 4 stage Johnson counter using 7474. Square wave can be converted to pulses using
a 2.2 k resistor in series and a diode in shunt to the square wave input.
16. Alamp iscontrolled by 4 switches. The lamp should glow under the following conditions
1. If any one switch is ON
2. If any two switches are ON. Set up a logic circuit.
Generate auuth table with inputs A. B. C. and D. Output willbe logic 1l for any combination
with only one input or two inputs are 1. Obtain the Boolean expression and realise the logic
circuit.
17. A lamp is controlied by 4 switches. It should be ON if andonly if one switch is ON. Realise
this using 8:1 MUX.
Generate a truth table and realise using MUX (using gates or 74151)
18. Atwo-soreyed building has one light in cach floor. Design acircuit using logic gates such a
way that anybody can switch ON or OFF both lights from any floor.
Let L be the lanp at ground floor, L be the lamp at first floor,
S,L be the swich at ground fioor to lamp L
S,L, be the switch at first floor to lamp Le
S,L be the switch at ground floor to lamp Land
SL be the switch at first floor to lamp L

First
floor

S Ground 1
floor

Truth table

Similarly L=SL@s,
Implement using two EXOR gates.
19. Generarte a 3-bit parity checker and if it is odd parity, generate its gray code.
Set up a3-bit parity checker and agray code generator. Parity checker output will be I
if the parity of input is odd. This output will activate the gray code generator. Use three AND
gates. One input to all AND gates should be from parity checker.
20. Design a ligh: system » hich is having 8 lights and each light should be ON for one second.
 321
Feed the output of 3-bit counter to a 3:8 decoder data select inputs. Clock must be IHz.
Afour storeyed building has one light on each foor. Design a circuit so that anybody can
switch on or off any light from any floor.
Let A be the light nn one floor. S,, S,, S, and S, are switches for light A on each floor.

s
SL Four identical cirCuits
one for each floor.

2, Set up a circuit to compare 3-bit numbers A and B. If A> B. perform additionA - B) and
if A< B perform subtraction (A-B).
The output of the comparator must be used as the control for 7483 adder'subtractot.
23. Design acircuit to count 0, 2, 4, 6,..98.
Set up a counter to count 0.1.2...99 using 7490 and short the LSB to ground to ensure
that the LSB is always at logic 0.
24. A counter has two control lines M and N. Design and impement the counter to count in the
following manner.
MN
0 0: 0, 1,2,3, 0...
0 1: 0, 1,0, 1..
1 0: 2,0, 2, 0..
1 1: 1,2, 1, 2..
25.Generate a square wave with duty cycle l/4, /2 and 34 from a nng counter
A square pulse signal with duty cycle 1/4 can be obtained shifung I through a 4 but rng
counter. Duty cycle can be made 1/2 and 34 respectively by shiftung ll and lll

when shatthng l

when saTtung ll

26. Set up a counter to count OH o CH

Iis a binary counter to count 0XXX0 to ll0
27. Design and set up alogic circuit o glow an tED iur tme swhen a key n pressed onee
This can be done by using athnmsLahle nuliv theaix folow ing an astabie msltiy ihratos
ucino 555 timer. ON iMe and off ume ot the aslate tmuttI ibrator should be l seood The
 322

output pulse duration of the monostable should e Ssoconds. The output of the monostabl
muBtivibrator must he conntad to the st pin of the astable multivibrator instead of connectiny
e tpin to V The astable output willbe available onlywhen the monostable multivibrator
cutput is high
28. Consider that there is a welland an overhead tank with levels marked as shown in figur.
Set up a water level controller to switeh ON and OFF a motor used to pump water from well
for the follow ing conditions.
1. Motor should be ON only if the water level in the tank goes below L, and water in
well is abve L.
2. Motor should be OFF if either the water level in the wellgoes below Lor water level
in tank is above L.

wÁl Tank

See the truth table given below. Mstands for motor. 0 for OFF and 1l for ON. At the input
side Oindicates below the level and 1 indicates above the level. Set up a combinational circuit
using gates. L L L, M
0

1 X

For example. first line in the truth table shows that the motor is OFF when the water is
below the levels L. L, and L,.
29. Design a BCD counter which counts 5 - 24.
Use 74192 and start the count from 5 by making use of load pin.Use a logic circuit to
reset after 24.
30.Design and set up an octal counte.
Use 7490 to count O to 7.
31. Design a counter to count from 00 to 77.
Use two 7490 ICs and reset both ICs at 7.
32. Set up a sequence counter using aMUX to count the sequence 0, 1, 3, 6...
The circuit diagram is shown in figure.
 323

Y
D, M
D'
M 4:1 MUX
Is,js,
(0 D, The 2 bit up counter counts the
D Y
M
D, sequence 00, 01. 10 and Ii When QQ,1s
00. all the MUX gives out the data at D. to
Y. When QQ, is 01. all the MUX gIves
D, out the data at D and so on.
D,
D, M,

Clock
|2 bit up counter

3. Generate the sequence 010011 using a MUX.

Set up a 8:1 MUX with data select lines driven by a mode 6up
counter Feed the
required sequence bits 01001lto data inputs to D, through D, of the MUX
34. Generate the sequence 1,3,5,7 by using an
asynchronous counter.

Q
+5 V

J, Here. LSB should aws

Clock 7476 7476
high. For that. Q, is tid to -5 \.
K, K

35. Generate the sequence 0,2,4,6 by using an asynchronous counter.

Here, lSB should e ahw ays

low Frthat. is tid
T toGround.
+5 V

Q.
Clock 7470
 324

36. Design a 3-bit even/odd counter using mode control.

Q.
EolQ, Q
+5 V J.
1 Even/Odd Q,
7476 7476
Clock

0 K,
1

1 When mode bit is 1, it counts 0,2,4,6,0 and

when mode bit is 0, it counts 1, 3, 5, 7, 1.

37. Design a circuit to count 2, 3, 6, 7, 2 using a single IC.

We can see that the centre bit of all the given numbers is 1. Take this as Q, whch is tied
to +5 V. MSB and LSB are Q, Q, which can be obtained from a two bit asynchronous
counter.

38. Implement a simple traffic light system for a four way junction.
When one junction is open others are closed. This is done using a 4-bit ring counter with
red LEDs are connected to Qoutputs and green LEDs are connected to Q outputs. At any
time one red LED and 3 green LEDs will glow.
39. Set up acircuit to devide an input frequency by 6.Output should have 50% duty cycle.
Use Johnson conter.
40. Set up a burglar alarm.
Use 555 astable multivibrator making use of V terminal.
41. Design a circuit which divides the input frequency by 10 and its output should be a square
wave with 50% duty cycle.
Use 7490 which has mode 2 and mode 5 counters in it. First, the input should be fed to
the mod 5 counter in it and its output should be fed to mod 2 counter.
42.Generate the following waveform
Input

Output
Set up a divide by 10 circuit as in previous question and feed the input and output to an
AND gate.
43. Set up an ASK generator using 555.
ASK generator is nothing but a gated astable multivibrator. Output of one astable
multivibrator (designed for low frequency) is connected to the reset pin (pin no. 4) of another
 325
astable multivibrator (designed for high freguency) Actve ls input t0 pín nn 4 wl
the output. reses
L Design a circuit for FM
generation using 555
Set up an astable multivibrator using 555 and
feed a igw frequency si
control voltage pin through a capacitor.
5. Set up a circuit to multiply two 3 bit
numbers.
Use AND gates and 7483 to
implement it.
6. Set up a circuit to light an LED in the pattern l1001110. 1means
Omeans not glowing for 1 giowing for isecond ana
second.
Set up a sequence generator using an asynchronous mod-8 counter For this. make a
truth table with inputs as counter states and required bits as output. Realise a
logiccircuit. combia

Unsolved examination questions

I. Designa 4-bit parity checker using a DEMUX.
2. Obtain the patterm 10110 shifting continuously.
3. Measure the noise margin of 7402 and 7404.
4. Design a system to turn on an LED when the input BCD code is invalid.
5. Set up an 3-bit UP/DOWN counter.
6. Set up a 4-bit comparator using 7485 and if A> Bdo addition and ifA<Bdo suberaction.
7. Design a circuit to multiply a given 3 bit number by 4. Answer must be in BCD.
8. Design a BCD counter to count the multiples of 5 up to l00.
9. Design a circuit to count the number of successive ls in a 4-bit data.
10. Implement a stop watch.
I1. Implement a 2-bit serial adder.
12. Implement a 2 bit calculator.
13.Realise the Boolean expression f( A. B.C.D) - (1,2,4, 5,9, 15) usng a MUX
14. Generate a fibanoccisequence less than 20.
15. Set up a full subtractor using a MUX.
I6. Experinmentally find the 2's complement of the miror image of a 4-bit number.
17. Check yhether the input number is palindrome or no (Pahndrome Nhas sy metricai bit
pattern w.r.t centre bit. For example, 10D.
18. Implement a ring counter without using flip lops.
19, Implement an 8:1 MUX using two 4: MUXs.
20. Design acircuit to light 6 LEDs in sequence using minimum number of 's
21. Set up a sequence detector (pattern detecto) todetect a ll0l patem.