Question

The figure shows a circuit designed to control a lamp. For what positions of the switches $S_1$ and $S_2$ will the lamp be lit?

• A. $S_1$ - Closed , $S_2$ - Closed
• B. $S_1$ -Closed, $S_2$Open
• C. $S_1$ -Open, $S_2$Closed
• D. $S_1$ -Either open or closed, $S_2$ Either open or closed

Answer 1

Answer: (D)

The truth table of the NAND gate is as shown below:

$S_1$	$S_2$	Out put
0	0	1
0	1	1
1	0	1
1	1	0

where a logic level $0$ at the input, $S_1$ or $S_2$, corresponds to a closed switch (at $0V$ ) and a logic level $0$ at the output means the lamp will be lit. From the truth table, both switches $S_1$ and $S_2$ have to be at open position in order to light up the lamp at the output of the NAND gate.