Logical OR Gate

We have gone through the different logic gates or if you have not you can check them out in the different articles of this website. Now discussing about the logic gates the obvious question which comes to our mind is that what is their operation or rather what is there field of use.

Before knowing about the operation let us briefly discuss about what is an OR gate. OR gate is an device which has two inputs and one output and the output is 1 if any or both of the inputs are 0 and the output will be 0 only if the inputs are 0. The notation of an OR gate is given as

OR gate

OR gate


As we have already discussed that it has two inputs, in the picture A and B are the two inputs and it has only 1 output.
Now let us explain the operation of the OR gate logically. We have already gone through the basics of the fundamental logic of OR gate. Now to describe it through Boolean algebra we have to look through the truth table first.

As we have already discussed that it has two inputs, in the picture A and B are the two inputs and it has only 1 output.
Now let us explain the operation of the OR gate logically. We have already gone through the basics of the fundamental logic of OR gate. Now to describe it through Boolean algebra we have to look through the truth table first.

Truth Table

We can describe the operation of an OR gate through binary arithmetic also. From this point of view the operation of OR gate is similar to binary addition. So if we take the two inputs of an OR gate as A and B then the output will be equivalent to A+B. now the so called truth table will be like

Input 1 Input 2 Output
A B Y = A + B
0 0 0
0 1 1
1 0 1
1 1 1

Logical OR Gate

Logical OR Gate


The truth table clearly explains the logical operation of the OR gate and the circuit diagram given below totally supports the truth table. Now we will explain the truth table along with the diagram simultaneously. When the input A and B both are 0 then we can say that both the switches S1 and S2 are open and the output is also 0. Now when A is 1 and B is 0 then we can say S1 is closed and S2 is open so the signal transmits through only S1 and the output will be 1. Similarly when A is 0 and B is 1 then the output will also be 1. Finally when both A and B are 1 then S1 and S2 both are closed and the output will be 1. This is what we call OR operation.

OR gate is available in TTL and CMOS ICS, Where TTL is Transistor - Transistor Logic ICs and CMOS is Complementary MOSFET ICS. We will now discuss on the arrange of OR gate on both the ICs. In CMOS ICS. 4000 series is available in digital electronic 4071 is the CMOS IC number of inputs in each gate is two. 4075 is 3 - input OR gate and 4072 is 4 - input OR gate in CMOS ICs. A internal gate picture of 4071 can make you understand about this IC.

IC 4071

4071 is a 14 pin 1C as you can see where four or gates are fixed together having two inputs. Pin number 14 is tvcc where maximum SV DC supply is given which activates the IC. Pin number 7 is grounded. Pin 1 and 2 is the input for the first gate and 3 is the respective output for the first gate. Pin number 5 and 6 are inputs for the second gate whose output is pin number 4. For the third gate pin number 8 and 9 are the inputs whose output is pin number 10 for the last gate pin number 12 and 13 are the inputs whose respective output is at pin number 11. Thus how the IC looks internally for CMOS.

ic 4071
ic 7432

IC 7432

In TTL logic the internal arrangement differs from that incase of CMOS logics. In TTL logic 2- input OR gate IC number is 74LS32. popularly known as 7432. LS stands for low power schotty version. Again one more type of IC number is also available in market which is 74HC32 where HC stands for high speed CMOS version i.e. it has lower electric current consumption and wider voltage range. 74LVC32 is another low voltage CMOS version of the same. Let us have discussion on 74LS32 which is a TTL IC. In this IC there are fourteen pin. The internal gate diagram of 7432 can make your idea clear.
In this IC pin 1 and 2 are the inputs of first gate where the output is from pin number 3. Again pin number 4 and 5 are the inputs of second gate whose output is in pin 6. Pin 10 and 9 are the inputs of fourth gate whose output is at pn 8. The input of the last gate or fourth gate is pin 13 and 12 and pin 11 is its output. Pin 7 is ground and pin 14 is +vcc supply where again +Vcc supply where again +SVdC is the normal and maximum supply. One thing must be maintain at +SVDC. If the i/p voltage would be more than this it may cause damage to the IC.