Find the correct combination of A, B, C and D | vedclass

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(D) The $LED$ glows when it is forward biased, meaning the potential at point $P$ must be high $(1)$ and the potential at point $Q$ must be low $(0)$.

Vedclass · Accepted Answer

$1101$

Vedclass · Answer

(D) The $LED$ glows when it is forward biased, meaning the potential at point $P$ must be high $(1)$ and the potential at point $Q$ must be low $(0)$.Let the output of the first $NOR$ gate be $Y_1 = \overline{A+A} = \overline{A}$ and the second $NOR$ gate be $Y_2 = \overline{B+B} = \overline{B}$.The output at $P$ is the output of a $NAND$ gate: $P = \overline{Y_1 \cdot Y_2} = \overline{\overline{A} \cdot \overline{B}} = A + B$.For $P = 1$, we need $A+B = 1$, which means at least one of $A$ or $B$ must be $1$.The output at $Q$ is the output of a $NOR$ gate: $Q = \overline{C+D}$.For $Q = 0$, we need $\overline{C+D} = 0$, which implies $C+D = 1$, meaning at least one of $C$ or $D$ must be $1$.Checking the options:$A) 0100: A=0, B=1, C=0, D=0 \implies P=1, Q=1$ (No glow)$B) 0011: A=0, B=0, C=1, D=1 \implies P=0, Q=0$ (No glow)$C) 1000: A=1, B=0, C=0, D=0 \implies P=1, Q=1$ (No glow)$D) 1101: A=1, B=1, C=0, D=1 \implies P=1, Q=0$ ($LED$ glows).Thus, the correct combination is $1101$.

Find the correct combination of $A, B, C$ and $D$ inputs which can cause the $LED$ to glow.

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