Video: Evaluating the Input of OR Gates

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

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Video Transcript

The diagram shows an OR Gate. If input A is zero and the output is zero, what must input B be?

Okay, so, in this question, we can see that we’ve got an OR gate. And we’ve been told that input A on this OR gate is zero and the output is zero as well. Based on that, we need to find out what input B is. So, to answer this question, let’s start by recalling the truth table for an OR Gate.

So, here is that truth table. Now a neat trick for actually memorising this truth table is to realise why an OR gate is known as an OR Gate in the first place. The reason it’s called this is because if either one of input A or input B is one, then automatically the output is one as well. And we can see that from this truth table.

In the first row, we can see the input A is zero and input B is zero. And in this situation, the output is zero. However, in all of the other rows, at least one of the inputs is one. For example, in the second row, input B is one. In the third row, input A is one. And in the fourth row, both inputs A and B are one. And for all three of these cases, the output is one as well. And so, if we remember this statement, that if either input A or input B is one, then the output is one, then we can easily memorize the truth table for an OR gate.

So, based on the information we’ve been given at the question, we know that we’re looking for a row in which the input A value is zero and the output value is zero. And hence, we’re looking for the first row, input A is zero and output is zero as well. This tells us that the value of input B must be zero as well. So, we write down zero up here. And at this point, we’ve found the answer to our question. For an OR gate, if input A is zero and the output is zero, then input B must be zero as well.

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