Question

A researcher wants an alarm to sound when the temperature of air in his controlled research chamber rises above ${{40}^{\circ }}C$ or falls below ${{20}^{\circ }}C$. The alarm can be triggered by the output of a
A.NOT gate
B.AND gate
C.NAND gate
D.OR gate

Answer 1

This problem can be solved by basics of gates in which we need to carefully observe that by which gate output we can sound an alarm in more than ${{40}^{\circ }}C$ and less than${{20}^{\circ }}C$, and for normal conditions it remain silent.

Complete answer:
To rise an alarm on 2 different cases we needed a gate which can give a required output when at least one condition is satisfied for example,
$\begin{aligned} & {{T}_{1}}={{40}^{\circ }}C\to 1,0 \\\ & {{T}_{2}}={{20}^{\circ }}C\to 0,1 \\\ \end{aligned}$
When temperature is high input will be high and for low temperature input will be low which means in any case we need high output 1 to rise an alarm in both the cases so there is only one gate in options which satisfied the above condition which is OR gate, let see the working of OR gate for our required problem,
The table of OR gate is given by,

i/p A	i/p B	o/p
1	1	1
1	0	1
0	1	1
0	0	0

First and last row will not be used because at the same time it’s not possible temperature to be higher than ${{40}^{\circ }}C$ and less than ${{20}^{\circ }}C$, but for second and third row we can observe that it clearly satisfy our required condition like, when temperature raise from ${{40}^{\circ }}C$ ,$0\to 1$ third row will be used and for decrease from ${{20}^{\circ }}C$, $1\to 0$ second row will be used

Note:
NOT, AND and NAND gate will not be used because they will not give the required output according to our need to raise the alarm as they will give 0 as output or can give 1 as output in exceptional cases(like last row and first row in OR table)