OR Gate – Logic Gates Tutorial

An OR gate is a logic gate where the output goes HIGH (or “1”) if any of its inputs are HIGH. So if A OR B is HIGH, the output Q also becomes HIGH.

The logic or Boolean expression for an OR gate is which means:

If A or B is true, then Q is true

Truth Table

OR gates can have more than two inputs. But no matter how many inputs it has, it will always give out a HIGH or logic “1” if one of its inputs is HIGH.

2-input OR Gate Truth Table

Input A	Input B	Output Q
0	0	0
0	1	1
1	0	1
1	1	1

3-input OR Gate Truth Table

Input A	Input B	Input C	Output Q
0	0	0	0
0	0	1	1
0	1	0	1
0	1	1	1
1	0	0	1
1	0	1	1
1	1	0	1
1	1	1	1

IC Alternatives with OR Gates

If you want to experiment and build circuits with OR gates, you’ll find them in both the 4000 IC series and the 7400 IC series:

• 4071: Four 2-input OR gates
• 4075: Three 3-input OR gates
• 74HC32: Four 2-input OR gates (HC is the family, can also be LS/HCT/…)
• 74HC4075: Three 3-input OR gates (HC is the family, can also be LS/HCT/…)

These should all be available as hobbyist-friendly through-hole chips. Just make sure you buy the DIP package version.

Example Circuit: Testing an OR gate

Below is a simple practical circuit you can build to test how this gate works. A Light-Emitting Diode (LED) turns on when you press either of the buttons A OR B. The circuit demonstrates the basic operation of an OR gate.

More Logic Gates Tutorials