Combinational Logic circuits Exercises

Question 1:

Design a circuit that counts the number of 1's present in 3 bits inputs A,B and C. Its output is a two bits number x,y. 


Answer

Question 2:

A bank safe has 3 locks with a key for each lock. Each key is owned by a different person. In order to open the safe at least two persons have to insert their keys into the assigned locks at the same time.

a) Draw the truth table
b) Design the circuit using AND and OR gates.
c) Design the circuit using the NAND gates only.

Answer:
a)

b) and C)

Question 3
(A/L) Past Paper

A fan in a room can be on (1) or off(0). A control system is required to operate the fan efficiently with the following conditions/ functionalities.

The fan can manually be switched on or off.
The timer will be either on or off.
The sensor will detect whether the environment is cold or hot.

The fan will automatically be switched on when the timer is on and the sensor indicates the environment hot.

The following table assign Boolean values for the above conditions/functionalities.
Conditions/Functionalities Boolean value

• Fan switched on manually 1
• Fan switched off manually 0
• Timer is on 1
• Timer is off 0
• Sensor detects cold environment 1
• Sensor detects hot environment 0

1. Draw a logic circuit by using a combination of only AND, OR and NOT gate to implement the above control system.
2. Construct a truth table to represent the functionality of the above system.
3. Write a Boolean expression (not simplified) to represent the truth table constructed in the above question.

Answer 3