Q.
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?
Semiconductor Electronics: Materials Devices and Simple Circuits
Solution:
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 $0 V$ ) 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.
| $S_{1}$ | $S_{2}$ | Out put | |
|---|---|---|---|
| 0 | 0 | 1 | |
| 0 | 1 | 1 | |
| 1 | 0 | 1 | |
| 1 | 1 | 0 |