E3 LOGIC CIRCUITS
(A)
INTRODUCTION
Logic circuits are becoming increasing important in today’s modern world. They are used to
process information and present results in useful form eg, calculators and digital watches. Logic
circuits are formed by combining various types of logic gates, such as NOR and NAND gates. There
are two main types of circuit, namely analogue and digital. Analogue circuits process information by
continuing fluctuating voltage which is dependant on the input signal. Digital circuits operates on
two distinct states, ie “0” and “1”. For positive logic system, a higher voltage level denotes logic “1”
while a lower voltage level denotes logic “0”. In a negative logic system, it is however represented in
the opposite way. In this experiment, positive logic system is used.
(B)
EXPERIMENT OBJECTIVES
There are two main objectives in this experiment and they are as follows:
(i) to understand the application of JK flip flops as a binary ripple counter ;
(ii) to analyse the simple combinational circuit which consists of a sequential logic circuit in
driving a sevensegment display device with useful logic gates.
(C)
EQUIPMENTS & COMPONENTS
Equipment List
1. Digital multimeter (DMM)
2. Function generator (FG)
3. Dual display cathoderay oscilloscope (CRO)
4. Breadboard with supply of 5V
Component List
1. 5 X 1K
Ω
, 7 Comm. Res. Net. SIL 330
Ω
2. IC type 74LS76 Dual JK flipflop
3. IC type 7408 Quad 2 input AND gate
4. IC type 74LS02 Quad 2 input NOR gate
5. IC type 74LS48 BCDtoSevenSegment decoder/drive
6. 7Segement LED (common cathode) Display Device
(D)
THEORY
Logic Gates
Digital signal processing circuits are called logic gates. These gates work in a twovalue
logic system, the binary digit “0” for the low level voltage and “1” for high level voltage. NOT
(inverter), AND, NAND, OR, NOR, XOR and XNOR are the seven types of logic gates. Their
symbols and truth table can be found in AppendixB. Combinational logic circuits are built from
logic gates and they depend on present inputs.
Sequential logic circuits
Sequential logic circuits are different from combinational
circuits as the output depends on
the current inputs as well as previous output. It possesses valuable memory characteristic.
A
flipflop (FF)
is a bistable sequential logic circuit storage device. Flipflops have valuable
memory characteristic because it remains in one of the states until a signal changes it into the second
stable state. There are the two types of triggering, namely edgedtriggered and leveltriggered. The
flipflop used in this experiment is negative edge triggered and Is set to toggle mode. This means that
J and K inputs are both at logic level “1” and successive clock pulse will reverse the output logic
state repeatedly.
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 Spring '09
 Yo
 Logic gate, function generator, ripple counter

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