What is logic OR gate?

OR gate is one of the basic logic gates used in Boolean algebra and electronic circuits. This logic gate gives a HIGH output (1) result if one or more inputs to the gate are HIGH (1). Again, a LOW (0) output results, if all inputs to the gate are LOW (0).

Let’s consider an OR gate with two inputs A and B that generate output Q. The output, Q of this Logic OR Gate returns “0” value when both A and B inputs are at a logic level “0”. In other words if A= 0 , B=1 or A=1 , B= 0  or A=1 and B=1 , OR gate gives logic level “1” output.

The logic or Boolean expression for a digital logic OR gate is that for Logical Addition which is denoted by a plus (+) symbol, giving us the Boolean expression as follows:

                                 Q = A + B ;  Q = A OR B

There are two symbols of OR operation and a truth table to prove OR operation as follows.

Symbol of OR gate
OR gate truth table

SET theory is also helpful to explain this gate. A and B are two set can be used to define Union set. Q  ∈  A ∪ B .   According to OR operation    A ∪ B is true if A is true, or if B is true, or if both  A and B are true.

SET theory for OR gate

We can use an electric parallel switch circuit to explain OR function. Let’s consider an electric bulb as an output Q where two parallel switches A and B are inputs. If either switch A or B is ON, or both switches A and B are ON then Q is ON. In other words if both switches A and B are OFF then Q is OFF.

Circuit diagram for OR gate

2-input Transistor

For the construction of OR gate, we can use RTL Resistor-transistor switches connected together as shown below. The inputs are connected directly to the transistor bases. For the OR logic, the transistors are in parallel and the output is driven high if either transistor is saturated “ON”.

Transistor OR gate

OR from NAND and NOR gate

OR logic can also be made from NAND or NOR gates. With two one-input NAND gate and one two-input NAND gate is in combined, we can easily construct an effective OR gate.

Two NOR gate with one is two-input and the other is one-input in combined, we can construct OR gate effectively.

If OR gate is not available, NAND or NOR gate in combination can be alternative of OR gate. Yes, OR logic gate is possible, since NAND or NOR gate is an universal gate. The image is given below.

OR gate from NAND gate
OR gate from NOR gate

OR gate IC

OR gate IC behaves as OR logic gate. There are some or gate IC. The most commonly used OR gate IC is IC7432. It is made with four independent positive logic OR gates in a 14-pin DIP package of two parallel rows.

Some OR gate IC are as follows:

  1. 7432 -OR gate IC
  2. CD 4071 – Dual-Input OR IC
  3. CD 4072 Dual 4-input OR Logic IC
  4. CD 4075 Triple 3-input OR Logic IC
  5. CD 4072B CMOS Dual 4-Input OR Logic IC
  6. 74LS32 – Quad 2-input OR Logic IC

For example, we consider OR gate IC 7432. Four OR gates have been placed in this IC as following the image.

7432 OR gate IC
OR gate IC

Related posts :

  1. What is logic gate?