Apparatus:
Two semiconductor diode (SD-1), power supply (SV), resistance 4.7k, LED, 2 two-way keys.
Theory
The “OR” gate is an electronic circuit with two or more inputs and one output. The output is high or at I if any one or more input are high or at state 1.
The symbol of two input ‘OR” gate is shown.
The Boolean expression of “OR” gate is F=A+B
The truth table for “OR” gate is give a side.
| Input | Output | |
| A | B | F=A+B |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
In order to understand “OR” gate operation, we must know the operation of semi conduct diode under forward biased and reverse biased condition as given in experiment No. 15.
Procedure:
1. Assemble the circuit diagram on bread board.
2. Ground both input A and B with the help of two way key to make A-0 and B-0. Both diodes do not conducted. The voltage across R is zero 0. Hence output F=0. The LED will not give light. So first line of truth table is verified.
3. Keep the terminal A at ground state and make B at high voltage (5V) with the help of two way key so that A=0 and B=1 diode
is OFF and diode
is forward biased and in conducting state
is in conducting. State, so voltage across it’s negligible. So full voltage of 5V appears across R. Hence LED will give light Thus second line of truth table is verified.
4. Make A=1 and B=0 by reversing the conditions in a above step. 5(v) gets forward biased and does not conduct, so
For forward biased diode
so
So full voltage of 5V will appear across R. Hence F=1. The LED will give light. Thus the
row of truth table is verified.
5. Make A=1 and B=1 with the help of two way key. Both diodes
and D₂ get forward biased and conduct current. The voltage drop across diodes in negligible and full voltage of 5V will appear across R. Hence F=1, the LED will give light. Thus the
row of truth table is verified.
Precaution:
1. The diodes should be checked before use.
2. The end of the connecting wires should be cleaned.
3. Make all connections first drawing the circuit diagram.
Viva Voce
Q.1: What is a truth table?
Ans: A truth table shows the output behavior of logic gate for various input combinations.
Q.2: What is a logic gate?
Ans: An electronic circuit which performs the logic operation is called logic gate,
Q.3: Why is the “OR” gate is sometimes called any or all gate?
Ans: Because its output is high if any or all inputs are high.
Q4: When is output of “OR” gate is low?
Ans: When all of “OR” gate are low.
Q5: What is LED?
Ans: lt is a semi-conductor diode specially designed to give light when it is forward biased.