Video: Identifying Circuit Symbols

The diagram shows the circuit symbols for four logic gates. Which symbol represents an OR gate? [A] Option (a) [B] Option (b) [C] Option (c) [D] Option (d)

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Video Transcript

The diagram shows the circuit symbols for four logic gates. Which symbol represents an OR gate?

Okay, so we need to find the diagram for an OR gate. So let’s go through each one of the options one by one. Now the first option looks almost like the diagram for an OR gate. However, this little circle at the right-hand end of the gate just before the output means that this cannot be an OR gate because the circle means that the gate beforehand gets inverted. In other words, whatever the behaviour of this gate before the circle, the circle will take the output value in each case and then flip it. So if we were going to have an output of one, the circle will take it and turn it into zero and vice versa. And so, in reality, option a) is actually the diagram of a NOR gate, where it’s almost like an OR gate, but with a NOT gate after it. So this cannot be the answer to our question.

Option b) then, we see first of all that it’s got a very nice D shape to it. And secondly, it’s got that little circle at the right-hand end as well. Now, D-shaped gates are actually AND gates. And the way to remember that is the word AND has a D at the end. And hence, the AND gate is a D-shaped gate. However, remember what we said about the little circle earlier. It flips the output of the AND gate. And so, the whole gate including the circle is now a NAND gate, in other words a NOT AND gate or an inverted AND gate. And hence, this cannot be our answer either.

Moving on to option c) then, we can see that it’s a D-shaped gate without anything on the right-hand end. And so, this is a traditional AND gate. Hence, this is not the option we’re looking for. And then finally for option d), we can see that this is the shape of an OR gate. It’s almost a D-shaped gate but with a curved left-hand edge. And so at this point, we found the answer to our question. The symbol that represents an OR gate is option d) in this case.

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