Question

security system for a building has three ways of detecting an intruder.
1. Security camera (SC) which sends a 1 when an intruder is detected.
2. Motion sensor (MS) which sends a 0 when an intruder is detected.
3. Noise level detector (NLD) which sends a 1 when the noise level rises beyond
a certain threshold.

The system should work such that the alarm should sound when
• SC doesn’t detect an intruder but the MS and the NLD do
• Neither SC nor the MS detects but the NLD does
• SC and the MS detect but NLD doesn’t
• SC detects only
• SC and NLD detect but MS doesn’t
However, it has been observed, that when the NLD is off, the system does not sound
an alarm even if the SC and MS detect an intruder.
Analyze the circuit for the problem and fix it.
For the above circuit
1. Construct a Truth table of the current system and analyze, the issue.
2. Construct a Truth Table for the modified (fixed) system.

b) A manual switch controlled 8 floor elevator is designed to operate such that when the
master switch S=0, it moves up (0,1,2. . . ) and when the switch S=1, it moves down
(7,6,5. . . ).
However, it has been observed that the elevator is experiencing the following
problems:
1. When the S=1, the elevator instead of going down gets stuck on the current
floor and does not move. (for example, if it is at the fourth floor, it remains
there and does not move down).
2. When the switch is 0, the elevator goes up but skips the 2nd and 3rd floor.
Analyze the circuit and identify the problem and fix the issue.
Show the state table of the current design.
Show the state table of the new (fixed) elevator.

Answer 1

The error in the current system is in accordance with  the case 3 as as shown in the figure below, that is when SC and MS detects but NLD doesnt, the alarm should sound actually in this case, so it might be in the truth table and then design followed by it the mistake had happened. One important thing is that MS sends '0' when detects Intruder and so it has to considered such in the design.The truth table that is modified one as shown in figure and the circuit design with respect to this will solve the issue.

b)

the state table of the current system clearly shows what the error is when the input (switch S)= 0 then sequential circuit must count up actually and but it skips the state C (2) and D (3) during the upcount , and when S=1 then the sequential circuit must count down but it stucks at the current count, as we can see in the state table .The modified state table is shown above, In which  we can see that When S=0 it counts Up and S=1 counts down, the sequential circuit must be designed with respect to this modified truth table and will solve the issue.