Video: Evaluating the Output of OR Gates

The diagram shows an OR gate. If input A is 1 and input B is 0, what will the output be?

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Video Transcript

The diagram shows an OR gate. If input A is one and input B is zero, what will the output be?

Okay, so, in this question, we can see that we’ve been given an OR Gate. And we’ve been told that the value of input A is one and input B is zero. We have to use this information to work out what the output will be. So, to answer this question, let’s recall the truth table for an OR gate.

And here is that truth table. Now an easy trick to memorise this truth table is to remember that an OR gate is called an OR gate because if either one of input A or input B is one, then automatically the output is going to be one as well. And we can actually see that in the truth table. When both input A and input B are zero, then the output is zero. But in every other combination of inputs, at least one of them is going to be one.

So, in the second row, we can see that input B is one. In the third row, we can see that input A is one. And in the fourth row, we can see that both of them are one. And so, in these situations if either input A, or input B, or both are one, then the output is going to be one as well. And that’s why this is called an OR gate. So, that’s a good way to memorise the truth table. But let’s come back to the information we’ve been given in the question.

We’ve been told that input A is one and input B is zero, and we need to work out what the output will be. So, to do this, we actually need to look at the third row in this table, where input A is one and input B is zero. And this row’s actually telling us that the output is one. And hence, we’ve found the answer to our question. For an OR gate, if input A is one and input B is zero, then the output will be one.

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