Aim

To understand the working of NAND gate using npn transistor, p-n junction diodes and to design a circuit of NAND gate using npn transistor, p-n junction diode, LED, push button type toggle switches and suitable resistances.

Introduction

NAND gate is an Boolean operator based on the mathematical function of complement of multiplication.

It works on digital signal as its input which have only two states of voltages, low as 0V and high as 5V.

NAND gate consists of electonic circuit of two p-n junction diodes and npn transistor having two input digital signals which can take four different combinations on voltages, for each combination of voltages there is a predefined output according to complement of multiplication rule.

Apparatus used

p-n junction diode (IN 4007)

Npn transistor (547 b)

Circuit board

Wires

9V battery and battery connector

LED (5mm, 1.5V)

Resistors

Push button type toggle switch

Push buttons

Theory and working

1.)Working of p-n junction diode in forward and reverse biasing

2.)Working of npn transistor in different configurations

3.)Understanding the functioning of NAND gate

4.)Circuit diagram used in making of NAND gate and its working

When the push button is in ‘released state’ then terminal ‘2’ is connected to terminal ‘3’, when push button is in ‘active or pressed’ then terminal ‘2’ is connected to terminal ‘1’.

The push button connected to first diode is ‘button 1’ and the other one is ‘button 2’.

When ‘button 1’ is released and ‘button 2’ is released then it is equivalent to A(0), B(0). In this state, first and second diodes are forward biased, current flows through diodes to negative terminal of battery but does not flow through base of transistor, leading to cut-off mode of transistor and current flows from collector to LED leading to high output.

When ‘button 1’ is pressed and ‘button 2’ is released then it is equivalent to A(1), B(0). In this state, first diode is reverse biased and second diode is forward biased, current flows through second diode to negative of battery but does not flow through base of transistor, leading to cut-off mode of transistor and current flows from collector to LED leading to high output.

When ‘button 2’ is pressed and ‘button 1’ is released then it is equivalent to A(0), B(1). In this state, first diode is forward biased and second diode is reverse biased, current flows through first diode to negative of battery but does not flow through base of transistor, leading to cut-off mode of transistor and current flows from collector to LED leading to high output.

When ‘button 1’ is pressed and also ‘button 2’ is pressed then it is equivalent to A(1), B(1). In this state, first diode and second diode are reverse biased, current flows through base of transistor, leading to saturation mode of transistor and current flows from collector to negative terminal of battery through transistor, leading to no current through LED and low output.

Conclusion

The output of NAND gate for different combinations inputs clearly shows the expected output according to the truth table.

Sources of errors

Write yourself

Precautions

Do not connect any electronic component without resistance in series, it can burn the component.

Bibliography

NCERT textbook of class 12

Physics by S.L.Arora

Circuits available on internet