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.
