Question

A person waiting in front of a lift of the Fth foor of a multi-story building, could push the button ∆ (say B1 ) or ∇ (say B2 ) to select the direction, up or down that he wants to go. The signal m sent by the motion detector M and the signals b1 and b2 sent by the push buttons B1 and B2 respectively are as indicated below. m b1 b2 ∆ ∇ M B1 B2 m = 1 when the lift is moving up m = 0 when the lift is moving down b1 = 1 when B1 is pressed b1 = 0 when B1 is not pressed b2 = 1 when B2 is pressed b2 = 0 when B2 is not pressed A logic circuit has to be designed to meet the following conditions. 1. When B1 is pressed the lift stops at the Fth foor only if it is in upward motion. 2. When B2 is pressed the lift stops at the Fth foor only if it is in downward motion. (i) Taking m, b1 and b2 as the inputs and Q as the output prepare a truth table to satisfy above conditions. Consider Q = 1 represents the lift stops at the Fth foor Q = 0 represents the lift does not stop at the Fth foor (ii) Using the truth table, obtain a Boolean expression for Q and draw the relevant logic circuit. (you may use logic gates with three inputs.)

Answer 1