Logic Gates using Transistors and Switches Experiment

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 T₁, 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.