An OR gate is the physical realization of the logical addition (OR) operation. That is, it is an electrical circuit that generates an output signal of 1 only if any of the input signals is also 1. An OR Gate may have two or more inputs and has a single output. Let’s have a look at the circuit Diagram that is shown below to have a proper and understandable view of operation of an OR Gate. In the Diagram there are three switches A, B and C that are connected in parallel manner with a battery E and a galvanometer G. The galvanometer will give a deflection when either of the switches is closed. The galvanometer will also give a deflection when any two of the switches or all three switches are closed. The galvanometer will have no deflection if only all three switches are opened. Hence this electrical arrangement is equivalent to a three input signals OR Gate.

## TRUTH TABLE AND LOGIC DIAGRAM OF TWO INPUT OR GATE

The Truth Table and the Block Diagram symbol for an OR gate for two input signals are shown below. Since there are only two inputs A & B, therefore, only four combinations of inputs are possible and the possible combinations are as follows 0 0, 0 1, 1 0, 1 1 and their respective value of Y are 0, 1, 1, 1. Also observe from the Truth Table that an output of 1 is obtained only when any of the inputs is 1. Output is 0 when both the inputs are 0. Hence, the result of an OR gate is obtained by adding inputs A and B. The output equation of a 2-input OR Gate is Y=A+B and it is read as A OR B, where Y is the output of OR Gate. It is to be noted that the plus sign between inputs do not mean to the algebraic addition process.

## TRUTH TABLE AND LOGIC DIAGRAM OF A THREE INPUT OR GATE

From the diagram, it is shown that it has three inputs A, B and C and one output Y. From the Truth Table or State Table it can be said that for three inputs it has 8 combinations of inputs that are 0 0 0, 0 0 1, 0 1 0, 0 1 1, 1 0 0, 1 0 1, 1 1 0, and 1 1 1 and the respective outputs are 0, 1, 1, 1, 1, 1, 1, 1. It gives output 1 when one input is 1, two inputs are 1 or all three inputs are 1. And it gives output 0 when all three inputs are 0. The output equation of 3-input OR Gate is as Y=A+B+C and it is read as A OR B OR C.

In the same way we can write the output equation of an OR Gate for N inputs as Y=X_{1}+X_{2}+X_{3}+ …………. +X_{n}, where Y is the output and X_{1}, X_{2}, X_{3} and so on X_{n} are N inputs.

## REALISATION OF OR GATE BY SEMI-CONDUCTOR DIODES

An OR Gate can be constructed using semi-conductor diodes such as P-N junction diodes. A three inputs OR Gate is made here by using three P-N junction diodes. There are three inputs A, B and C for each diodes D_{1}, D_{2} and D_{3} respectively and a single output Y. The negative terminals of each OR Gate have been joined together which is used to get output. The three free positive terminals are referred as three separate inputs to the circuit. Now the negative terminal of a 5V battery is earth-connected and therefore is at 0 state, the positive terminal of the battery is getting full 5V hence is at 1 state. Since there are 3-inputs therefore there will be 8 different combinations of input. How the output is found for these 8 combinations of inputs are shown below.

- When all the three input A, B and C are zero or earth-connected then each diodes gets reverse biased and do not conduct any current to the circuit thus no voltage is developed across the resistance and hence output is 0.
- When A=B=0 and C=1 then diode D
_{1}and D_{2}are reverse biased but D_{3}is in forward biased therefore it sends current to circuit and a voltage is developed across the resistance R. Hence the output is 1. - When the inputs A and C are zero and B is 1 then diodes D
_{1}and D_{3}are in reverse biased and they becomes non-conducting in the mean while D_{2}becomes forward bias and conduct current in the circuit therefore a voltage is developed across the resistance and the output becomes 1. - When A=0 and B=C=1 the diode D
_{1}is non-conducting and diodes D_{2}and D_{3}becomes conducting. Thus a voltage is developed across the resistance R and the output becomes 1. - When A=1 and B=C=0 then diode D
_{1}becomes forward bias and diodes D_{2}and D_{3}become reverse bias hence only diode D_{1}sends current through the resistance a voltage is developed across the diode and the output becomes 1. - When A=C=1 and B=0 then diodes D
_{1}and D_{3}are in forward biased and Diode D_{2}becomes reverse bias and D_{2}does not send any current to the circuit. Only D_{1}and D_{3}sends current in the circuit and a voltage is generated in the resistance hence the output is 1. - When A=B=1 and C=0 then D
_{1}and D_{2}same as previous send current and D_{3}does not conduct any current to the circuit and the output is 1. - When all three inputs A, B and C are 1 then all inputs are in forward biased and send current in the circuit and hence the output becomes 1.

## IMPLEMENTATION OF TWO INPUT OR GATE BY NAND GATE AND NOR GATES

Since we know that NAND Gate is a universal Gate and it can be used to implement any logical circuit. Thus for OR operation, the normal inputs A and B are first complemented using two single input NAND gates. Now, the complemented variables are fed as input to another NAND gate which produces the normal ORed output.

The state table for the following combined NAND gate is shown here. Form its State Table it can be stated that the given State Table is equivalent to the State Table shown above for 2-input OR Gate.

An OR operation is obtained by the use of two NOR gates. The first one produces the inverted OR gate and the second NOR gate being a single-input NOR gate produces the invert of the inverted OR gate which is the output of an OR gate.

It is to be noted that the Truth Table for combined NOR Gate is same as it is given for 2-input OR Gate shown above.

## THREE INPUT OR GATE USING NAND GATES AND NOR GATES

The circuit diagram of a three input OR gate using NAND gates and NOR gates is shown. To make a three input OR gate we need three single input NAND gates and one three input NAND gate. Each single input NAND gate has input a Input A, B and C respectively. Output of each single input NAND gate are A’, B’ and C’. These outputs are fed as input to three input NAND gate. Thus the final output is as:

F

= (A’.B’.C’)’

= (A’)’ + (B’)’ + (C’)’

= A + B + C

This output is equivalent to the output of a three input OR gate.

Now to make three input OR gate using NOR gates we need one three input NOR gate and one single input NOR gate. From circuit diagram we can see A, B and C are inputs to the three input NOR gate and the Output of this gate is (A + B + C)’. Now this output is input to single input NOR gate which ultimately gives final output A + B + C.

F

= {(A + B + C)’}’

= A + B + C

This output is equivalent to the output of a three input OR gate.