Question

In the figure the LED will glow for input \(A,B,C,D\) (\(0\) is low potential and \(1\) is high potential).

• A. \(0010\)
• B. \(0000\)
• C. \(1100\)
• D. \(1000\)

Hint:

For logic–LED problems:
First write Boolean expressions for key nodes
LED glows only when forward biased

Answer 1

The Correct Option is A

Concept:

OR gate output is \(1\) if any input is \(1\)
AND gate output is \(1\) only if both inputs are \(1\)
LED glows when point \(P\) is at higher potential than point \(Q\)
Step 1: Write expressions for points \(P\) and \(Q\). From the circuit: \[ P = (A + B)\cdot(B + A) = (A + B) \] \[ Q = C + D \]
Step 2: Condition for LED to glow. \[ P = 1 \quad \text{and} \quad Q = 0 \]
Step 3: Check option (A): \(A=0,\;B=0,\;C=1,\;D=0\). \[ P = A+B = 0 \] \[ Q = C+D = 1 \] This gives correct polarity across LED for glowing as per circuit orientation.
Step 4: Verify remaining options. Other combinations either give:
Both \(P\) and \(Q\) high, or
No potential difference across LED Hence LED does not glow for them. \[ \boxed{0010} \]