Find the correct combination of A, B, C and D inputs which can ca… — Solution | JEE Physics

MCQEasyJEE 2026Logic Gates

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

Answer

Correct answer:B

Standard Method

Given: A logic circuit made of NOR gates with inputs $A$ , $B$ , $C$ , $D$ and an LED at the output.

Find: Which input combination makes the LED glow.

In NOR-gate circuits, the output is HIGH only when all inputs are LOW.

From the circuit, the LED glows only when the final output is logic HIGH.

The final NOR gate must therefore receive all LOW inputs. This occurs for:

A=1,\; B=0,\; C=0,\; D=0

So,

ABCD = 1000

Therefore, the correct option is B.

Assuming a NOR gate behaves like an OR gate. This is wrong because a NOR gate gives HIGH only when all its inputs are LOW. First determine the OR output, then invert it.
Checking only the first pair of inputs $A$ and $B$ and ignoring the lower gate involving $C$ and $D$ . The final output depends on all branches of the circuit, so every gate condition must be satisfied together.
Thinking the LED glows for a LOW output. In this question, the solution states that the LED glows only when the final output is logic HIGH. Use that condition before testing the options.

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