1. Logic Gates using Transistors (NOT gate)
Aim
To verify the truth table of NOT gate using a transistor
Apparatus
An npn transistor, 5 V battery, two resistances,
two way key, voltmeter, soldering iron and lead, etc
Theory
It is a single input gate for which the output is always the
complement of the input. Complement of 1 is zero and that of zero is 1. The
complement of A is indicator A' (A prime) or Ā (A bar). Logic gate symbol
for NOT gate is shown in the figure. The truth table for NOT gate is also given.
A

Y = Ā

0

1

1

0

Procedure
Connections are made as shown in the diagram. When A is
earthed (A = 0), the base of the transistor is earthed. Since there is no
input, the output current l_{C} = 0. The transistor is in cut off mode.
The output voltage is 5 V; i.e., Y = 1.
When A is put in contact with the terminal T_{1}, it
is connected to the positive terminal of the battery, the base emitter voltage
is 5 V; i.e., A = 1. Now, the collector current I_{C} flows. The values
of R_{C} and R_{B} are so adjusted that the maximum collector
current flows. The transistor is said to be in saturated state. Then the output
voltage, Y ≈
0.
Result:
The truth table of the NOT gate is verified.
2. Logic Gates using Switches (OR, AND, NOT)
Aim
To verify the truth tables of OR, AND and NOT gates using
switches.
Apparatus
A 2 V accumulator, 470 Ω resistor, two switches, LED,
etc
Procedure
(a) To verify the truth table of OR gate
Connections are made as shown in the diagram. The switches A
and B are connected in parallel. The switches A and B are kept open; i.e.,the
inputs, A = 0, B = 0. The LED does not glow; i.e.,the output Y = 0.
The switch A is kept closed (i.e.,input A = 1) and the
switch B is kept open (B = 0). Then the LED
glows, i.e.,the output Y = 1.
When the switch A is open (A = 0) and B is closed (B = 1),
then the LED glows (Y = 1). When both switches are closed, A = 1 and B = 1,
then also LED glows (Y = 1)
Thus the truth table of the OR gate is verified.
(b) To verify the truth table of AND gate
The connections are made as shown. The switches A and B are
connected in series. The switch A and B are kept open; i.e., A = 0 and B = 0.
The LED does not glow; i.e.,Y = 0. The switch A is closed (A = 1) and B is kept
open (B = 0). Then it is seen that the LED does not glow (Y = 0).
Now, the switch A is kept open (A = 0) and B is closed (B =
1). The LED does not glow (Y = 0). When both the switches are closed (A = 1, B
= 1), the LED glows (Y = 1).
Thus the truth tables of AND gate is verified.
(c) To verify the truth table of NOT gate
The connections are made as shown. The switch A is connected
in parallel to the source. The switch A is kept open (A = 0). The LED glows (Y
= 1). When A is closed (A = 1), the LED does not glow (Y = 0). Thus the truth
table of NOT gate is verified
Result
The truth tables of OR, AND and NOT gates are verified.
The Logic Gates Viva Questions with Answers
(i) What are the basic logic gates?
Ans: Basic logic gates are AND, OR and NOT.
(ii) Why are the NAND and NOR gates called digital building
blocks?
Ans: They can perform all the functions of the basic logic
gates. So they are called digital building blocks (or universal building
block).
(iii) NOT gates is called inverter, why?
Ans: Its output is the complement of its input. That is, its
output is the inversion of the input.
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