Question

An electric circuit conducting a power source, a bulb and two switches $(S_1,S_2)$ is shown in the figure.


The above circuit is equivalent to which of the following logic gate circuits ?

• A. $\text{NAND}$
• B. $\text{NOT}$
• C. $\text{AND}$
• D. $\text{OR}$

Answer 1

The correct option is D $\text{OR}$

We can observe from circuit, that the bulb will light up $\left(ON\right)$ if current flows in circuit.

Thus, if $S_1$ is closed, $S_2$ is open and vice versa, circuit is still completed and bulb turns $ON$

$S_1$ closed $\Rightarrow A=1$
$S_2$ Closed $\Rightarrow B=1$
$S_1$ Open $\Rightarrow A=0$
$S_2$ Open $\Rightarrow B=0$

$\Rightarrow A=1,B=0,Y=1$

$\Rightarrow A=0,B=1,Y=1$

$\Rightarrow$ If both the switches are closed, then also the current will flow in circuit.

$\Rightarrow A=1,B=1,Y=1$

Bulb will turn $OFF$ if both switches are open.

$\Rightarrow A=0,B=0,Y=0$

Thus, we can observe that the output is $1$ if any one of the inputs is $1$.

$\therefore$ The given circuit represents $\text{OR}$ gate.

Hence, option $\left(D\right)$ is correct.

Why this question? If intends to test your understanding of binary system in real application. Tip : To depict every phenomenon in binary system, concentrate on the fact that everything has to assigned values in terms of Yes $\left(1\right)$ or No $\left(0\right)$ / High value $\left(1\right)$ or Low value $\left(0\right)$.