Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 8

Physics Past Exam • 2017/2018 • Pack 1 • Question 8

04:37

Video Transcript

Using only cells, switches, and bulbs, draw a diagram for an electric circuit which represents each of the following types of logic gate, where switches represent inputs and bulbs represent outputs. 1) A NOT gate. 2) An OR gate.

Considering how we can make an electrical circuit that represents a NOT gate, let’s start by drawing out the truth table of this operation. For this operation, we have an input marked 𝐼 and then we have our output which is the result of the NOT operation on the input. If our input is true, that means the NOT operation gives us false and if our input is false, that means the NOT operation gives us true. It reverses our input.

When it comes to making a circuit that represents this, we’re told that bulbs represent our output and switches represent our input. Starting with the bulb, we’ll say that if the bulb is lit, that corresponds to an output of true and if the bulb is not lit, that corresponds to false.

We want to wire up this bulb with a switch in such a way that if the switch is true, the bulb is false and if the switch is false, the bulb is true or turned on. We can do that by putting our bulb and a switch in parallel with our power cell. And we’ll see that our switch when it’s open as it is now corresponds to an input of false, while when it’s closed, that corresponds to an input of true.

Let’s test this circuit we’ve built now and see if it agrees with our truth table. Considering how current moves in this circuit, when the switch is open, that means no current can travel through that branch. So it all needs to pass through the branch with the bulb in it. And as the current passes through the bulb, it causes the bulb to light up. So when we have an input of false, our switch is open and we have an output of true, our bulb is lit.

Now, we’ll reset our circuit and let’s consider what happens if our switch is closed. As the current flows from our power cell and reaches this parallel branch, the resistance is close to zero in the straight wire. So almost all the current will take that path and almost no current will flow through the bulb, which has a very high resistance. So it will not light up. In this case then, we have an input of true our switch is closed and an output of false our bulb is not lit up. This means that our circuit does accurately represent a NOT logic gate.

Next, let’s work on designing an electrical circuit that represents an OR gate. Like before, we’ll start with the truth table of this logic operation in this case we have two inputs: input one and input two along with our output, the OR operation on these inputs.

When input one is true and input two is false, our output is true. When input one is false and input two is true, our output is again true. When both inputs are true, our output is true as well. And only when both inputs are false, do we receive a false output.

We’ll once again start building our circuit with our bulb the output which when it’s lit represents an output of true and when it’s not lit represents an output of false. In this circuit, we want to design it so that if either of our inputs is true, our output is true as well. To do that, we’ll put two input switches in parallel with one another.

Calling this top switch input one and the bottom switch input two, let’s test out whether this circuit follows the OR gate operation. When both switch one and switch two are open, as shown now, current is unable to flow to the circuit. So the bulb doesn’t light up. That agrees with the last row of our truth table.

On the other hand, if we close switch one while leaving switch two open, that means current is able to flow through switch one’s branch of the parallel section. The current passes on through the light bulb and beyond lighting it up. This outcome agrees with the first row of our table.

Next, we open up switch one, but we close switch two. This corresponds to input one being false, but input two being true. With switch two closed, we’ve created a path for current to flow. This current once more moves through the bulb and lights it up. So we see there’s agreement with the second row of our table as well.

And finally, we close both switch one and switch two, corresponding to inputs of true and true. Now, current is able to flow through both branches of the parallel circuit, passes on through the bulb, and lights it up once more.

This indicates the circuit we’ve designed does operate as an OR gate. Using cells, switches, and bulbs then, we’ve created a circuit that represents a NOT gate and a circuit that represents an OR gate.

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