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SYSTEMS PREPARATION QUESTIONS 2008 - 8

V. Ryan © 2008

 

The Theme Park owner has invested in an automatic height detection system. This checks that potential riders are the safe height for each ride. A person who is too small could easily slip through safety barriers leading to a serious accident. Anyone too small for the ride is directly to the exit. Medium and tall people are also separated so that they can be distributed equally throughout the carriages.

 

The sensors detect the height of each person and are connected to two control circuits. One circuit controls barrier X and the other barrier Y. If the output of either of the control circuits is high (logic 1) then the appropriate arm will swing to the position of the dotted line.

 
     

A. Complete the truth table below to show the logic states of the three sensors that would give the correct outputs. The sensors provide a ‘high’ (logic 1) if they detect a person and a’ low’ (logic 0) if no person is detected.

 
ANSWER

Part of each control circuit is composed of AND gates.

In the space below draw a diagram that represents an AND gate. Also, complete the truth table (opposite) for a two input AND gate.

ANSWER
Draw a circuit diagram to show how two of these gates could be used to control barrier ‘Y’

The inputs from sensors A, B and C are already drawn along with the output - barrier ‘Y’
ANSWER

The control system has been found to be unreliable. It has been modified by using sensors which give logic 0 when they detect a person and logic 1 for NO person.

What type of gate could be added to your circuit (drawn above) to allow for this change?

 
   

ANSWER

NOT GATE (INVERTER GATE)
 
Draw the modified circuit in the space below

Suggest a more cost effective modification to the one shown above.
 

ANSWER

A three input AND gate followed by a NOT gate would by more efficient. Or a three input NAND gate.
   
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