As its name implies, an OR logic gate performs an ”OR“ logic operation, which is an addition. It has at least two inputs. So, if A and B are its inputs, at the output we will find A + B. So, OR logic gate can be summarized by the formula Y = A + B. You can see its symbol in Figure 10 and its truth table right below it.

Figure 10: OR logic gate.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	1

Another way to understand OR logic gate: its output will only be at ”0“ when all its inputs are also at ”0“. Otherwise its output will be ”1“.

If you need more than two inputs, the same idea applies. If you are projecting a circuit and need an OR logic gate with more inputs, you can go ahead and simple draw an OR logic gate like the one in Figure 10 and put more inputs on it. But if you are working with integrated circuits with OR logic gates with fewer inputs that you need, you can expand the number of inputs by connecting them like shown in Figure 11.

Figure 11: Expanding OR inputs using 2-input gates.

The most famous OR gate integrated circuit is 7432 and you can its pinout in Figure 12. Of course there are several other integrated circuits that provide OR gates with more inputs. For example, 7427 provides three three-input OR gates.

click to enlarge
Figure 12: 7432 integrated circuit provides four two-inputs OR gates.